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.[1] He devised mathematical problems for his contemporaries to solve.

Examples

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)

References

  1. Editors of Time-Life books, Inc. (1989). The Puzzle Master. Alexandria, Virginia, USA: Time-Life Books. p. 18. ISBN 0-8097-0928-7.