How high will they go ?

A kangaroo at the Sydney zoo kept getting out of his enclosure every night. Knowing that mature kangaroos could hop very high, the zoo officials replaced the eight-foot fence with a ten-foot fence. He was out the next morning, just roaming around the zoo. They tore down the ten-foot fence and put up a fifteen-foot fence.
He was out again the next morning.
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, "About a thousand feet, unless somebody thinks to lock the gate at night!"