And of course it has a simple solution. It is inconsistent. But the story is not really that simple. The story is a retelling of a problem in set theory. In set theory, we have sets, collections of objects.
Some sets contain infinitely many elements, as the set of all even numbers.
A set can contain sets. It also contains the empty set, by the way. All sets contain the empty set. We can define the set of all sets. Some sets contain themselves.