First we fill a bowl with water and then we put some apples in the water.
Because apples are less dense than water, they float at the surface.
Now we are ready to play. Children stand in a circle around the bowl of water with their hands behing their back. They try to catch one apple with their teeth. They cannot use their hands: sometimes they are tied behind the back to prevent cheating.
There is a variation of the game: the apples hanging on a string, instead of in a bowl of water. This is called "Apples on a string".