Two kangaroos

Two kangaroos were living in a zoo pen with a 20-foot high fence.
One morning, both kangaroos were found wandering around the zoo, and were quickly put back in their pen, and the fence was put up to 30 feet.
Next morning, they were found outside again, so the fence was made 40 feet high.
The fence got up to 60 feet, and still the kangaroos were outside in the morning.
One kangaroo says to the other, “How high will they make this fence, do you think?”
“Don’t know,” says the second. “Depends when they discover they’re not locking the gate.”