Newton's cradle is a device that demonstrates the principles of momentum and energy conservation. It consists of a series of suspended metal balls, typically five, that are aligned in a row. When one ball is lifted and released, it strikes the next ball, transferring its energy and causing the last ball on the opposite end to swing outward. The motion continues as each ball transfers energy through the line. If one ball is lifted and released, one ball on the opposite end will swing out with the same speed. This simple yet effective demonstration illustrates fundamental concepts in physics.