Answer: Climbs the water tower
Arnie, Gilbert's younger brother (played by Leonardo Di Caprio), is mentally challenged, and has some eccentric habits. One of them is climbing the water tower if left unsupervised. Gilbert has met a girl named Becky, and forgets to monitor Arnie closely because he is so deeply engaged in conversation with her. Arnie's arrest is the catalyst for forcing his mother to leave the house for the first time since her husband died, so that she can get him released from custody.