DS5 just finished reading book "The Cat in Numberland" by Ivar Ekeland. I highly recommend it to any child interested in infinity.
It's a story based on Hilbert's idea of explaining infinity using a hotel with infinite number of rooms, one number per each room.
All the natural numbers are happy, each number has its own room and the hotel is full. Then one day number 0 comes and wants a room too. They show how to fit it in the infinite hotel. Then 26 letters come and they can still fit in the infinite hotel. Then half of the numbers leave but the numbers can still be rearranged and the hotel stays full. Finally fractions come as well and they too fit in the Hotel Infinity.
