Self-assembly of nanoparticles is like putting together a bunch of tiny pieces of a puzzle by themselves. Nanoparticles are really small pieces that can be made up of molecules, atoms, or even other particles. When the nanoparticles are put in the right conditions, they can join together and form larger structures. It is like if you had a bunch of small lego blocks and water, the blocks would float around and eventually start to join up with one another and make something like a nice big tower.