At Mr. and Mrs. Hilbert's Hotel Infinity, the resident cat is puzzled. The hotel is fully booked -- the rooms are full of Numbers, both Odds and Evens -- yet guests continue to arrive. When Zero shows up, there's a massive room switching, and he stays, even though Mr. Hilbert insists he's not really a Number. Then the Letters appear and everyone still has a room, even though no Numbers have left. No matter how many guests arrive or depart, the hotel accommodates them all, and is always full. But when the Fractions arrive, demanding rooms, real chaos threatens. Can an ingenious solution be found to house them all? Based on mathematician David Hilbert's famous paradox of the Grand Hotel, "The Cat in Numberland" offers a refreshingly clear explanation of infinity for readers of all ages. John O'Brien's imaginative line drawings further elucidate the complex concept, as well as simple functions like addition, subtraction, division, and multiplication.