system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does some plastic and some nail polish hardens with UV?
gpt
Here is one way to do it....simplified ELI5.

The chemical you use is just a little bit unstable.  Molecules of this chemical want to be attached to other molecules but they need a little push of energy.  THat push of energy can be UV light (other types of light don't have enough energy to 'activate' the molecule).  

The UV light causes the molecule to get active enough to attach to another molecule.  If you do this enough, you wind up with all of the molecules hooked to each other in a giant mesh, so its one big solid network rather than lots of smaller individual molecules.