You may be familiar with matryoshka dolls: nested sets of painted figurines that fit within painted figurines. In the case of wooden dolls, the concept is pretty straightforward: hollow out a large bit and fit smaller bits in. You might think that doing the same thing with atoms is kind of tough. Yes, atoms are mostly space, but they are kind of difficult to hollow out. And, convincing another atom to enter the empty space (and make itself smaller) seems an impossible task.