A whistle is a small device that produces a high-pitched sound when air is forced through it. Typically made of metal, plastic, or wood, whistles are commonly used in sports, signaling, and safety applications. They can vary in design, with some featuring a pea inside to create a distinct sound. Whistles are often associated with referees in sports like soccer and basketball, where they signal fouls or stoppages in play. They are also used by lifeguards to alert swimmers of danger and by train conductors to signal the approach of a train.