Zoo Problem

A kangaroo kept getting out of his enclosure at the zoo. Knowing that he could hop high, the zoo officials put up a ten-foot fence. He was out the next morning, just sauntering around the zoo. A twenty-foot fence was put up. Again he got out.
When the fence was forty feet high, a camel in the next enclosure asked the kangaroo, “How high do you think they’ll go?”
The kangaroo said, “Unless somebody locks the gate at night, I think they will just keep going!”