They're your tootsies, your dogs, your piggies. Whatever you call them, feet are an important part of you. Without them, you couldn't stand up, walk around, or run a race. In fact, your feet work so hard for you that sometimes they get sweaty. And stinky.
Think of a hot, summer day when you've been walking around an amusement park all afternoon. Oooh, boy — your feet have been in those sneakers a long time! On the car ride home, you decide to kick off your shoes. It feels good, but it smells bad. In fact, you might get an earful from the other passengers in the car: P.U. — what stinks?
Bacteria are to blame. These tiny critters normally inhabit your feet and love dark, damp places like the insides of sweaty shoes. They multiply in sweat, so if you don't wear socks, that really gets them going.
In the right conditions, bacteria will feast on your feet. These bacteria eat dead skin cells and oils from your skin. Their colonies will grow and start getting rid of waste in the form of organic acids. It's those organic acids that smell bad.
And for 10% to 15% of people, the smell is really bad. Why? Because their feet are extra sweaty and become home to bacteria called Kyetococcus sedentarius (say: kite-oh-KAH-kus SEH-den-tair-ee-us). These bacteria produce more than just stinky organic acids — they also produce stuff called volatile sulfur compounds. Sulfur compounds usually are powerful and awful smelling. If you've ever smelled a rotten egg, you know what volatile sulfur compounds smell like.