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As a 7th grade science teacher, I’ve spent the past eight years or so learning to mold science content into a form that 12 year old kids might enjoy. As a result, I find myself catering to their rather juvenile sensibilities. For example, 7th graders find farts absolutely hilarious! And that is why I work them into my material whenever possible. Wait, no! Not actual farts. I don’t stand at the front of the room shootin' down fairies or anything (That's a euphemism that I ran across while researching for this article, and I had to work it in). But, there are actually quite a few scientific concepts that can be taught using farts.

States Of Matter

We all know the states of matter: solid, liquid, gas, plasma, Bose-Einstein condensate, neutron-degenerate matter, and quark gluon plasma. Okay, maybe you’re not familiar with those last few, but they only occur under extreme laboratory conditions, so that’s excusable. In 7th grade science, we just stick to the basic four. We learn that a solid is a substance with a definite shape and definite volume. A liquid has definite volume but no definite shape. In other words, it can take the shape of the container it is poured into. Gas doesn’t have a definite volume or definite shape. It can expand to completely fill up its container. Finally, plasma is superheated gas that becomes ionized and gives off energy in the form of light.
For the purposes of this article, we are going to focus on gas: both the state of matter and the flatulence. If you’ve ever farted in the middle of a crowded room, then you understand how farts obey the definition of a gas. At first, only you and the people immediately surrounding you can smell it. The molecules that make up that fart cloud are all pretty concentrated in a small space. But, it begins to spread through a process called diffusion. Diffusion is the passive movement of particles from an area of high concentration to an area of low concentration. As the gas cloud spreads, more people begin to smell it. It is in the process of filling up its container: the room. But there is a limit to how far it can go; or so it seems. Eventually, the smell goes away. We usually just assume that the fart is completely gone, but it is really still lingering. So why can’t we smell it anymore? That question brings us to the next scientific topic we can teach using flatulence.

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How We Smell

Your sense of smell is called your olfactory system. It, along with your sense of taste, is a part of the chemosensory system. The chemosensory system is made up of sensory neurons that can detect certain molecules from their environment and translate them into an electrical signal that the brain can process. Most sensory neurons are specialized so that they can only detect a specific compound, which is mind blowing when you realize that your olfactory system can detect up to 10,000 different smells! But, where do these smell-inducing molecules come from?

If a substance contains volatile compounds that can easily evaporate or form tiny particles that can float in the air, then that substance probably gives off a smell. Farts are (hopefully) made entirely of gases to begin with, so they definitely give off a smell. When that unholy mixture of noxious fumes squeeks past someone’s sphincter, they begin to spread out into the room, as was mentioned above, and the poor unfortunate souls left in the wake are pretty much forced to breathe it in through the nostrils and past the olfactory neurons located deep within your sinus cavity. When some of the compounds contained in the fart - specifically methane, ammonia, and hydrogen sulfide - bind to their specific sensory neurons, they fire off a deluge of electrical signals which olfactory bulb translates into an entire symphony of smell.

Think about this for a minute:

When you smell a fart, it means that actual molecules that were locked up inside someone’s colon escaped through their rectum and are now clinging to nerve cells INSIDE of your sinus cavity!

I guess I could have fried some bacon in a pan and got the same concepts across, but that’s not nearly as hilarious!

I’ll Just Hold My Breath Until It Goes Away

That’s the thing: it kind of doesn’t go away. Not for a long time, anyway. This is our chance to talk about concentrations and threshold levels in the body.
Of course, the smell of the fart eventually fades away, but that doesn’t mean that the poo particles are gone. You see, the brain tends to ignore a lot of signals until they reach a certain intensity, and your sense of smell is no different. You can have fart chemical binding to some of your olfactory neurons, but if there aren’t enough of them firing at once, the brain simply ignores the signal. For example, Hydrogen sulfide, the gas responsible for rotten egg farts, has a smell threshold of .03 - .05 ppm. So, out of every 100 million molecules that you breathe in, if fewer than 3 of them are H2S then your brain won’t register the smell. That sounds acceptable... until you do the math:

Assuming you’re at sea level and room temperature, the air around you contains about 2.5 x 1023 molecules for every 1 mL of volume. Written out in longhand, that would be:

250,000,000,000,000,000,000,000

Now, realize that you inhale up to 3000 mL of air each time you inhale!
That means you inhale about 750,000,000,000,000,000,000,000,000 molecules in each breath!
With a smell threshold of .03 ppm, you could breathe in something like 250,000,000,000,000,000 (250 quadrillion!) poop tainted H2S particles and never even know it!
Remember this the next time someone across the room farts, but the smell doesn’t make it to you. You man not smell it, but the particles are probably still there; just not in a high enough concentration for your brain to pay attention to.

And for goodness sake, DON’T BREATHE THROUGH YOUR MOUTH!!