A kangaroo kept getting out of his enclosure at the zoo. Knowing that he could hop very high, the zoo officials put up a ten-foot fence. He was out the next morning, just wandering around the zoo. A twenty-foot fence was put up. Again he go 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, “Pretty high, unless somebody figures out to lock the gate at night!”