Two cylindrical pieces of wood laid across the top of the three stumps at each end of the wicket. Grooves are set into the top of the stumps to prevent the bails rolling off or being blown off by the wind. For a batsman to be given out by being bowled, one or both bails must fall off the stumps. If the ball hits the stumps and the bails stay on, then the batsman cannot be out. The same applies to a run out.