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Michael: Have you ever taken a good, long look at your arms? You might've noticed that they're similar to your legs in a lot of ways: in both your arms and legs, there's a single bone in the top half connected to two bones in the bottom half by a joint that only bends back and forth. But there's at least one key difference: legs have kneecaps, whereas elbows don't have any kind of cap.

The reason why is actually pretty simple – you don’t need elbow-caps because you don’t walk on your hands. Your knees face forward and bear your weight when you walk, so they need a little extra help and protection. The kneecap – or as biologists call it, the patella – is a sesamoid bone, a bone that’s only connected to tendons instead of other bones.

One of those tendons is connected to the quadriceps, a muscle on the front of your thigh that does the work when you extend your knee. A tendon crosses over the knee joint and anchors the quadriceps just below the front of the knee, so that when the muscle contracts, your leg straightens.

The kneecap positions the quadriceps muscle’s tendon farther out from your knee joint, so that extending your knee takes less force — like how a longer lever needs less force. This makes your quadriceps muscle 30% more powerful than it would be otherwise. The patella also helps protect your knee joint and make sure the tendon flexes correctly under the stress of walking.

While the kneecap is definitely the biggest sesamoid bone in your body, it’s not the only one – smaller ones near your big toes help your feet bear your weight. So what about elbows? In your arm, the equivalent of the quadriceps is the triceps muscle on the back of your arm, which straightens your elbow. The triceps tendon crosses elbow joint and attaches to the ulna, the bone of the pinky finger side of your forearm. Prongs on the ulna help keep the tendon in place, but there’s no sesamoid bone involved. Elbows just don’t need as much help as knees to stay functional, since they don't bear as much weight.

Animals can gain and lose sesamoid bones through evolution depending on their needs – a few animals, like some bats and lizards, do have sesamoid bones in their elbow joints. So the next time you try to lick your elbow, now you'll know why you aren't licking an elbow-cap.

Thanks to Patreon patron Matt Mills for asking this question, and thanks to all our patrons, who keep these answers coming. If you’d like to submit a question to be answered, just go to Patreon.com/SciShow. And don’t forget to go to YouTube.com/SciShow and subscribe!