A boy is stuck on a deserted island. There is a bridge to connect the island to the mainland. Halfway across the bridge there is a guard. The guard will not let anyone from the mainland to the island, or anyone from the island to the mainland. If the guard catches someone, he sends him or her back. The guard sleeps for 30 seconds and then is awake for 5 minutes. The island is surrounded by man-eating sharks, and the boy does not have anything with him except for his own shirt and his pants. It takes the boy 1 minute to cross the bridge. How does he cross the bridge without getting caught?
The boy runs halfway across the bridge and turns around. The boy looks like he is running to the island and is sent to the mainland.
What runs but never walks, has a mouth that never talks?