“What has a bottom at the top?” sounds impossible if you picture an object sitting normally on a table. The trick is that “bottom” can refer to part of the human body rather than the lowest part of an object.
Answer: Your legs.
Your bottom is located at the top of your legs, so the wording is literally true even though it initially sounds contradictory. The riddle works by making you interpret “bottom” as a position before revealing that it means a body part.
It is a classic example of a simple riddle where changing the meaning of one familiar word completely changes the solution.
