How Many Senses do Humans Have?

People are generally familiar with the five basic senses: sight, smell, hearing, touch, and taste. However, it’s less well-known that humans have closer to 21! Aristotle is credited with the traditional “five senses” model, and science has advanced a little bit since that time. There can be some debate about what a “sense” actually is: the common definition is “any system that consists of a group of sensory cell types that respond to a specific physical phenomenon and that corresponds to a particular group of regions within the brain where the signals are received and interpreted.” Here are the senses humans have (or at least the ones discovered so far!)

  • Sight – color: sight is actually two senses. Color is detected with the cone receptors in the eye.
  • Sight – brightness: detected with rod receptors
  • Smell: based off a chemical reaction, and separate from taste, though the two combine to produce flavor. This is why food tastes funny when you have a cold.
  • Sound: detecting vibrations along a medium that is in contact with your ear drums, such as air or water. The density of the medium affects the vibrations reaching your ear, which is why things sound different underwater.
  • Touch: This is an interesting one, because there is a unique touch sense, separate from your senses of pressure, temperature, pain, and itch. Your skin is the largest organ on your body and it gives you lots of data!
  • Taste: This can be argued to be five senses, each based off chemical reactions with the taste receptors on your tongue. There are different receptors for sweet, sour, salty, bitter, and umami (which detects the amino acid glutamate, often found in meats).

And here already we can see the beginning of the debate. I only listed the “five” traditional senses, and depending on how those are counted, they could be as many as 14!

  • Pressure: something is pushing on a part of you. This has to be differentiated from detecting atmospheric pressure changes, because that is still debated. There’s plenty of anecdotal evidence that humans can detect changes in atmospheric pressure – becoming sleepy or experiencing joint pain in low pressure periods – but there’s nothing conclusive.
  • Itch: this is actually distinct from other touch-related systems! Evolution has provided us with some strange things.
  • Thermoception: Ability to sense heat and cold. This can be counted as two senses, because the body has different types of thermoreceptors for detecting external and internal temperature.
  • Nociception: sense of pain. This was originally thought of as overloading other senses, but in fact it is a separate system. There are three different types of pain receptors: cutaneous (skin), somatic (bones and joints), and visceral (body organs).
  • Proprioception: my favorite sense, and one of my favorite words. The sense of where your body is without looking at it. This is how you know where your foot is even if you aren’t watching it move.
  • Tension: sensors in your muscles monitor how much tension they are under
  • Stretch: receptors found in the lungs, bladder, stomach, and intestinal tract, to tell when those organs are full. A type of stretch receptor that senses dilation of blood vessels is involved in headaches.
  • Equilibrioception: this is what your inner ear is for. Detects balance, acceleration, and directional changes. Without this sense you can’t tell which way is up – it’s a little important!
  • Hunger: self explanatory. Your body says to give it calories! The brain can actually sense the difference between different macronutrients, and the body has some learned appetites which explain a craving for, say, salty foods, when you are short on sodium.
  • Thirst: my rule of thumb is that if I feel thirsty, I should have been drinking already…
  • Chemoreceptors: involved in detecting blood-borne hormones and drugs. Can also detect carbon dioxide levels in the blood: the brain uses that information to control breathing rate.
  • Magnetoception: the ability to detect magnetic fields. Not as strong as in many other animals like birds, but experiments have shown humans do have some sense of direction in this way. This can be tested by placing a person next to a strong magnetic field and disorienting them. People in this scenario are much worse at re-orienting themselves in terms of the earth’s magnetic field than people who are not near a strong magnetic field.
  • Time: No singular mechanism has been found to explain how humans tell time, but there is conclusive proof that this sense is startlingly accurate. There seem to be different mechanisms for short term (minutes to hours) and long term (circadian rhythm) time keeping.
    One interesting fact about humans’ time sense is that it is dependent on age: people within the age range of 19-24 years were able to tell within 3 seconds when 3 minutes was up, whereas those in the age group 60-80 thought 3 minutes had passed when the actual time was 3 minutes and 40 seconds. So as you get older, it’s not that the world is speeding up; it’s that you are slowing down!

It’s always a source of joy for me that there are so many things we still do not know about our own bodies. It’s easy to get jaded and think that science has answered every question, but the fact that we still have so many gaps in our knowledge is tremendously gratifying. We’ll never run out of new things to learn!


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2+2 = 5

My mom has had a piece of paper hanging up next to her desk for years now, saying:

2+2 = 5*

*For very large values of 2

It never made sense to me until recently, when I actually thought to look it up (shock!). Obviously, 2+2 is 4, not 5: 2 is a constant and can’t have a different value. It’s actually a joke, playing on the ideas of rounding and estimating. Say your calculator is set to show numbers to zero decimal places, so it only displays a whole number. Then if you entered 2.48 the calculator would display it as 2. If you add 2.47 (which would also only show as 2) to that, the actual answer is 4.95, which the calculator will round to and display as 5. So, 2+2=5 for large enough values of 2!

The joke is also a reminder about being aware of these rounding errors in real life. Engineers deal with this a lot, which is why we tend to approximate π as 3.14 (or 3 if we’re being really lazy!). Any number in real life has to be estimated: at some point, the measurement has to be cut off. The danger comes in rounding all your numbers, then doing the calculation, because this can lead to a final answer that is quite far away from the one you should have come up with. Rounding errors compound surprisingly quickly, and it’s something to be aware of.

In school, we usually got around this issue by storing all the values in our calculator (to however many digits it could handle) and only rounding the final answer. This usually ensured we were as close as possible to the right answer. In real life, that’s not always an option, which is when you need to start tracking significant figures. A scale that weighs to 0.1lb can’t be used to estimate to the 0.01 of a pound: it’s only accurate to the first decimal place. So, if you’re adding that weight to the weight from a scale that is accurate to 0.01lb, you still have to keep your final answer to the 0.1lb, because you can’t use more significant figures than your least accurate measurement. This ensures you’re not incorporating rounding errors into the math.

So, a very nerdy joke with a good reminder about the difficulties about existing in the real world!

What’s your favorite nerdy joke? Even better if it comes with a lesson!


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Idea Generation Techniques

I’m working for an innovation consulting group now, so the question of where ideas come from has been on my mind lately. I’ve had some opportunities to see these methods formalized before, both in a conference I attended during college, and in a Design for Six Sigma course I took last year. In both these areas, it was emphasized that brainstorming is a really inefficient method for idea generation, once the process is actually quantified (usually in terms of ideas per minute). The human mind works best when it can generate connections between different concepts, and use those connections to make new ideas. So, these formalized methods try to help people generate new ideas by giving them a framework they can use to make those connections. Here are the methods:


Brutethink: forces you to see relationships between dissimilar things. This technique will encourage you to see ideas where none existed before.

1) Select a random word (nouns are best). Random words from unrelated contexts are a rich source of connection-making material. The best words are simple and familiar so you can easily visualize the objects they represent.

2) Identify things that are associated with your chosen word. What are its characteristics? What does it do? What does it contain? What words or actions come to mind?

3) Draw connections between your chosen word and its characteristics, and your challenge. Generate ideas from these connections. Develop concepts from your ideas.

For example: select a random word, like “tree”. Things associated with the word (and this is a partial list, of course): trunk, branches, roots, leaves, shade, rake leaves, grow, grain, vertical, needles, swing, prevents erosion, forest, fertilize, water, color, shape, height, colorful, pine cones, seeds, harvest, mill, etc. Then,  you can grab a word from that list and see how it inspires you in the problem you’re trying to solve.


Check Listing: starts with an existing situation and asks, through the use of action words, how you might modify the situation to make it different and better. Or, it can be used as a follow-up to brainstorming, to improve new ideas. Action words to try:

Adapt: What else is this like? What can be copied from something else? What other uses could it be adapted to?
Magnify: Add to it. More time, strength, height, width, duplicate, and exaggerate.
Modify: Change the color, shape, size, weight, texture, or energy source, etc.
Minimize: Subtract, split, condense, reduce, remove, lower, lighten, shrink.
Substitute: What or who else? A different part, ingredient, approach, or process
Rearrange: Interchange components, other patterns, layouts, sequences, direction, speed.
Reverse: Use opposites, transpose, reverse roles and actions.
Combine: Units, purposes, elements, components, ingredients.
Remove: Parts, processes, functions, elements, components.
Other Uses: Are there other functions or purposes to which this can be applied?


Double Reverse / Pain Storming: This creativity technique has us initially reverse our typical thinking, and identify ways to make the situation / challenge worse. Then, it asks us to reverse these ideas, to make things better. Hence, the name Double Reverse. So, if your process is running too slowly, think of ways it could be made to run even more slowly, then reverse those changes to speed it up again. Can any of those changes be applied to the current situation?


TRIZ: Theory of Inventive Problem Solving. A man named Genrich Altshuller, while studying Russian patents, noticed the same principles were used over and over again to solve similar problems. TRIZ is a compilation of the most effective solutions to technical problems, independent of industry.

This is a huge topic and I’ve only seen a small part of it. The tools available are Contradiction Table, Inventive Principles, Separation Principles, Problem Formulation, Ideality / Resources, Function Analysis, and Technology Evolution. Information on all these is available online.

Technical and Physical Contradictions: at the heart of many problems is a contradiction between two requirements. These contradictions can be either: technical, where alternative solutions improve one aspect of the design at the expense of another, or; physical, where the physical state of the object must be in two states at once. An example of a technical contradiction would be power vs. fuel consumption: when one goes up, the other must go down. An example of a physical contradiction would be car suspension: it should be hard, for good handling and control, but also soft, for a comfortable ride. Within each technical contradiction, there is at least one physical contradiction. The idea of using the TRIZ techniques is to avoid having to make these trade-offs by breaking the contradictions. Here is a website to use the technique: you select the type of technical contradiction you’re running into, and it returns principles or techniques to solve the issue.

In addition, to help solve physical contradictions, you can try using the Separation Principles, which are as follows:

1) Separation in Time: a parameter or element of a system is present or absent at different times.
2) Separation in Space: a parameter or element of a system is present or absent in different “spaces” or spatial orientation.
3) Separation in Scale: a parameter or element of a system is present or absent depending on the scale you view it at. Also, between parts of a system and the whole.

Again, TRIZ is a huge topic and could merit a post on its own. From what I’ve seen, it’s a tremendously powerful tool for idea generation and problem solving.


The key thing about idea generation is that creativity is a skill that can be developed with practice. Brainstorming is not the best technique to use, especially if you haven’t had much practice at it (want to get better? Pick a common object and spend five minutes thinking of as many common or off-the-wall uses for it as you can), so these techniques are a great way to jump-start the process and get you and your group thinking in new directions.

In addition, using these techniques forces you to spend a solid amount of time thinking about the problem, instead of spending two minutes on it and declaring it impossible. It does a lot to encourage you to really stretch your brain: have you really thought this through? Is that really the only way to solve the problem?

Have you used any of these techniques? Want to give them a try? Talk about a problem you’re trying to solve!


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Sustainability Topics: LEED Certification

Companies looking for ways to show they are being more sustainable often turn to certifications for their buildings. Buildings use up a lot of energy, during the construction phase and especially during daily use, so being able to show that companies are using less energy and are thinking about sustainability is a big deal. Over the next few weeks, I’ll be describing some of these certification programs in more detail, starting with LEED:


LEED (Leadership in Energy & Environmental Design) is a certification program for all building projects, not just new construction. It is overseen by the US Green Building Council (USGBC). Projects satisfy prerequisites for their type, and earn points within the program to achieve different certification levels.

Depending on the project type, five different rating systems can be used: Building Design and Construction, for new construction or major renovation; Interior Design and Construction, for interior renovation; Operations and Maintenance, for existing buildings undergoing improvement work with little or no construction; Neighborhood Development, for land development projects involving multiple buildings; or Homes, for single or multi-family, low- to mid-rise homes. The points available for a project depend on the rating system being used.

There are different credit categories in which projects can pursue different types of credits to earn points. Categories include Location and Transportation, Water Efficiency, Sustainable Sites, and Innovation. The categories have prerequisites that must be satisfied, and then more points can be earned on top of the baseline.

The number of points a project earns determines its certification level: 40 points is enough for basic certification. LEED Silver, Gold, and Platinum require 50, 60, and 80 points, respectively.

People can also be LEED certified, by taking the LEED credential exams. This shows you have expertise in the area of certification. The first level is Green Associate; after that, you can take LEED AP and specialize in a specific project type.


So, at the start this looks like a great program. It’s easy to follow and see how many points you’re earning, the prices are reasonable for obtaining certification, and the program is very popular – in Washington D.C., all new public buildings must achieve LEED certification. However, it is not without controversy. Critics say the system is too easy to manipulate, by earning easy points and not making actual substantive change. In addition, there’s no requirement for energy use models to line up with actual usage, and there is some debate about whether LEED buildings actually use less energy than similar non-LEED buildings. Also, there is little emphasis on using the building sustainably: having lights that turn off automatically when no one is in the building doesn’t help if people stay there working through the night. Finally, the USGBC has no way to take certification away if the project doesn’t live up to its models. Once a building has been certified, it is always certified, even if it’s using more energy than it should be.

LEED was the first green building design program to become really popular, though that doesn’t mean it should stay popular if there are better programs. I’ll be exploring some of these alternatives over the next few weeks. It is important that LEED paved the way for green building design to become a standard in the industry, and we should be looking for ways to improve these programs to enforce better standards for construction.


Let me know what you think – is LEED still a helpful program? What’s your favorite standard for green design?


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Blood Types

I can always remember my blood type, for a very silly reason: I’m A+, and in school that’s the grade I always aimed for. Today, I want to learn about what all these labels mean – what makes our blood different?


At core, everyone’s red blood cells are the same. What is different is the molecules attached to the surface of these blood cells: antigens and proteins. A person can have either A- or B-type antigens, or both, and which type a person has is determined by genetics. A certain gene can code for production of type A, type B, or no antigens. And, since a person has two copies of the gene (one received from each parent), they could have one copy coding for type A and the other copy coding for type B. This is what determines your blood type, as shown in this picture:

blood type inheritance

So, my red blood cells have type A antigens. This means the genes I got from my parents coded for A and A, or A and O.

The + or – next to a person’s blood type is determined by the presence or absence of the Rh protein on the red blood cells. A “+” means the person does have that protein, while a “-” means they do not. So, I am type A+: I have the type A antigen and the Rh protein on my cells.


This gets tricky when we talk about donating and receiving blood. A person will have antibodies in their plasma determined by whichever antigens are not on their cells. The antibodies are designed to detect and attack any foreign molecules that may have entered the body from outside: for example, blood with different antigens on it. I can accept blood from someone with type A antigens and Rh proteins, but not from someone who has type B antigens: my body would detect it and start an immune response.

So, someone with blood type AB+ can accept blood from anyone: they don’t have any of those antibodies in their plasma, so their body will recognize any blood as their own. They can only give blood to other AB+ people, though. On the other hand, someone with blood type O- can give blood to anyone, because their blood has none of the antigens or proteins that could trigger an immune response. They can only accept blood from other O- people, though, since their own plasma contains every relevant antibody. The chart below shows who can donate to and receive from whom:



So, it’s simpler than it looks. Two different antigens, and one protein, can code for eight different blood types. In the US, type O is the most common, while AB is the least common.

This gets me all excited about genetics again! My favorite part of that class was charting inheritance across generations. There’s something very satisfying about figuring out the likelihood of the different combinations…

So, what’s your blood type?

– Hannah