Bobbing for apples (known in some places as ducking for apples) is a game customarily played on Halloween. The game is played by filling a tub or a large basin with water and putting apples in the water. Because apples are less dense than water, they will float at the surface. Players (usually children) then "bob" or "duck" for the apples, by trying to catch one with their teeth. Use of hands is not allowed.
History
The custom originated among the Celts as part of their celebrations of the Samhain season, celebrations that grew into our modern Halloween. The original custom generally took place during large gatherings of people. Apples, which were associated with love or fertility goddesses, were placed either in a tub of water or suspended from a string, and unmarried attendees would attempt to catch the apple in their mouths. According to tradition, the first person to catch the apple would be the next one to marry -- a tradition that echoes the modern "throwing of the bouquet" at weddings.