The surprising answer offered by the quiz is B. the belly button. Its shape can create a small, protected space where sweat, natural skin oils, lint, dead skin cells, and microorganisms collect. Because the area can sometimes be overlooked during bathing, it can support a surprisingly diverse microbial community.

Research examining belly-button microbiomes has indeed found considerable bacterial diversity, although that doesn’t establish the belly button as scientifically the single “dirtiest” body part. The answer depends entirely on how “dirty” is defined. The mouth, feet, skin, and other areas each contain distinctive microbial communities, and simply having more types of bacteria doesn’t mean an area is unhealthy.

There’s usually no reason to worry about these microorganisms. Bacteria naturally live throughout the human body, and many are harmless or play useful roles. Basic hygiene is generally enough: wash regularly, gently clean and dry the belly button, brush your teeth twice daily, clean between your teeth, and keep your feet clean and dry.

So if this particular quiz defines “dirtiest” as an area capable of hosting a surprisingly diverse collection of skin bacteria, the intended answer is the belly button. But the bigger lesson is even more interesting: bacteria are a normal part of being human. Did you guess the belly button, or were you convinced the answer was feet or teeth?