Hilbert's paradox of the Grand Hotel
Now suppose that there is a hotel that has an infinite number of rooms. As a convenience, the rooms have numbers, the first room has the number 1, the second has number 2, and so on. If all the rooms are filled, it might appear that no more guests can be taken in, as in a hotel with a finite number of rooms. This is wrong, though. A room can be provided for another guest. This can be done by moving the guest in room 1 to room 2, the guest in room 2 to room 3, and so on. In the general case, the guest in room n will be moved to room n+1. After all guests have moved, room 1 is empty, and the new guest now has a room to occupy. This shows how we can find a room for a new guest even if the hotel is already full, something that could not happen in any hotel with a finite number of rooms.