The kangaroo at the local zoo was somehow managing to escape every day, so the zookeepers constructed a 10-foot tall fence around his enclosure to keep him in.
The next day, the zookeepers found he'd escaped again, so they built a 20-foot tall fence around the enclosure.
Again, the kangaroo escaped, so the zookeeper contructed a 40-foot wall with spikes at the top and electrical wire along the bottom to prevent him from jumping out.
In the next enclosure, the giraffe asked his friend the kangaroo, "How high do you think they'll keep building this wall?
The kangaroo replied, "I don't know, maybe a thousand feet. Either that or until someone stops leaving the gate open at night."