Replace the question mark with a number

A fourth-grade teacher was giving her pupils a lesson in logic.

"Here is the situation," she said. "A man is standing up in a boat in the middle of a river, fishing. He loses his balance, falls in, and begins splashing and yelling for help. His wife hears the commotion, knows he can't swim, and runs down to the bank. Why do you think she ran to the bank?"

A girl raised her hand and asked, "To withdraw all his money from his savings account?"