The answer to the riddle "What has a head and a tail but no body?" is "a coin." This riddle is popular due to its counterintuitive nature.
The front side of a coin is often referred to as the head, partially because coins usually display a portrait of a historically significant person. The opposite side of a coin is referred to as the tail because the tail is the opposite of the head when referring to living creatures. The riddle is tricky because most things having these qualities are living beings, which have bodies in addition to heads and tails.