A simple multiple-choice question has gone viral online, leaving people debating the same topic: Which is the dirtiest part of the human body—A. Feet, B. Belly button, or C. Teeth? At first glance, the answer seems obvious. Many people choose feet because they spend hours inside shoes, where sweat and odor can build up. Others choose teeth because the mouth contains an enormous community of microorganisms.

But the question becomes more complicated when you ask what “dirty” actually means. Feet can collect sweat, dead skin cells, bacteria, and fungi, while the mouth contains hundreds of microbial species. Most of these microorganisms are normal, and many are harmless or beneficial. So visible dirt, odor, and microbial diversity aren’t necessarily the same thing. The third option hides in a small area many people barely think about while showering—and it’s the answer this viral quiz is looking for. Continue below to discover why.