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If you've spent any time trying to explode a pumpkin with rubber bands you know that they have a distinct smell to them when stretched, and you have physics to thank for that.

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If you’ve ever spent a few hours wrapping things up with rubber bands, like, if you were trying to explode a watermelon or a pumpkin or something, you know that distinctive smell that rubber bands let off when you stretch them. That scent is the result of the weird physics of rubber, because stretching a rubber band actually heats it up.

This is something you can feel for yourself at home. Take a rubber band, stretch it quickly, then gently touch it to your upper lip. I say your lip because lips actually have a higher density of temperature-sensing nerve fibers than your fingertips.

Anyhow, it’s slightly warm, right? Now let the rubber band relax quickly, but carefully! Don’t fling it at anyone’s eye or something.

And the touch it on your lip again. It’s much cooler, and that’s because of something called entropy. Although entropy can be a bit of a scary term, for our purposes, it simply refers to the amount of disorder in a system.

If you zoom in to the molecular level, rubber bands are made of long chains called polymers. When the rubber band is at rest, these polymers are disorganized, kind of like a big pile of spaghetti. But when you stretch the rubber band, the spaghetti strands become straighter and more organized, more like a box of straight, uncooked spaghetti.

And because the rubber band is more organized, it has less entropy. But the second law of thermodynamics states that the entropy in a system can't decrease over time. So, to maintain the same amount of entropy when the polymers are stretched out, the rubber band has to become more disordered in some way.

And that way is by some of the molecules in the rubber band moving faster. When molecules move faster, their temperature is higher, hence the warmer feel to the stretched-out rubber band. Now, this also happens to make the rubber band smell more.

And that’s because higher temperatures cause substances to have more volatility: basically, they evaporate more easily. For example, water at room temperature doesn’t evaporate very fast. Its volatility is low.

But if you heat that water up enough, it starts to turn into steam. Its volatility becomes high. Water doesn’t have a strong smell, but the same principle applies to odorants, chemicals that do have a smell to us.

Natural rubber contains dozens of odorants, including things like limonene, a lemony scent found in many citrus fruits, and trimethylamine, a compound with a strong fishy smell. You might not think rubber bands smell lemony or fishy, but mixed with all the other smelly molecules freed by the heat of stretching, you get that distinct, rubbery smell. Thanks for asking!

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