A very large mathematical convention was held in Las Vegas. The conventioneers filled two hotels, each with an infinite number of rooms. The hotels were across the street from each other and were owned by brothers.

One evening, while everyone was out at a bar-b-que, one of the hotels burned to the ground. The brothers got together and worked out a plan. In the remaining hotel, they moved all guests to twice their room number — room 101 moved to 202, room 1234 moved to room 2468, etc.

Then all the odd number rooms were empty, and there were an infinite number of odd rooms. So the guests from the other hotel moved into them.