A brain teaser is a form of puzzle that requires thought to solve. It often requires thinking in unconventional ways with given constraints in mind; sometimes it also involves lateral thinking. Logic puzzles and riddles are specific types of brain teasers.
One of the earliest known brain teaser enthusiasts was the Greek mathematician Archimedes. He devised mathematical problems for his contemporaries to solve.
Q: If three hens lay three eggs in three days, how many eggs does a (statistical) hen lay in one day?
A1: One third. (Note: 3 hens = 3 eggs / 3 days → 3 hens = (3 / 3) (eggs / days) → 1 hen = (1 / 3) (egg / days))
A2: Zero or one (it's hard to lay a third of an egg).
(information from wikipedia)