The riddle
What Has 5 Toes but Isn't Your Foot?
Who is speaking?
Everything turns on two small words. Merriam-Webster defines "I" as the person speaking and "you" as the person spoken to, so the same foot gets a different name depending on who is talking. The teller's foot is "my foot" to the teller, and the listener's foot is "your foot". By saying "isn't your foot", the teller rules out the listener's feet and leaves the teller's own. The answer belongs to the teller.
Counting toes
A human foot has five toes: the big toe, which anatomists call the hallux, and four smaller ones, ten toes in all across two feet. That is why the riddle works at all, because any ordinary foot satisfies the description and the only thing left to decide is whose foot it is. Merriam-Webster lists hallux as the word for the big toe.
A riddle every teller answers differently
Passed around a group, this question gives a different foot to each person but the same words every time: "my foot". Each teller owns a different foot, yet all of them are right. The words never change, but the foot does.
Guesses that miss
Most listeners start by hunting for an animal with five toes. That is a natural first move, since the phrase "but isn't your foot" sounds like a hint to look elsewhere. The usual answer is the teller's own foot. A listener who gets stuck can be nudged with "ask who is talking", which turns attention from the object to the person.