Proceeding naturally from there: two is a set of two elements. It contains an element that is an empty set and an element that is a set containing an empty set. Three is a set of three elements: empty set, set containing an empty set, and set containing a set containing an empty set. It's as simple as that. I'm sure you're all capable of projecting further numbers.”
A murmur ran through the class as the students muttered to themselves, endeavouring to grasp the new concept. The professor held up his hand.
“I have a riddle for your homework. While it's true that I believe sets are far more interesting than numbers, I want you to prove that all natural numbers are also 'interesting'.
As soon as the class was over, our four friends agreed that they needed to go to the library. They raced along the path, full of anticipation, but as soon as they entered the building, they realised there was an obstacle. The strict Librarian was lurking near her desk, ready to pounce. She looked them up and down, registering their uniform.
“Naturals are forbidden to enter the library because they are known to deface the books. They are only allowed in the company of a Teacher.”