system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do glow-in-the-dark things work?
gpt
Imagine a piston, with a hole at the bottom to let fluid in. Pump in a bunch of water, and the piston rises up; let go, and the piston will drop, squirting water out of the bottom of the piston. 

With glow in the dark materials, this piston is an electron, and the water is photons (light particles). When a photon hits an electron in a material, that electron gains a bunch of energy - like the piston rising up - and the photon is absorbed. When the electron eventually "falls down" back to its original energy level, it releases a photon at a particular wavelength (color). 

Now, pretty much every material in the world does this, but glow in the dark stuff is special. When that electron rises up in energy, it gets stuck there - quantum mechanics says it shouldn't be able to fall back down, although it eventually will anyway. That's why glow in the dark material can "store" light for a while - the electrons are stuck in a high energy state, and slowly drop back down (releasing light) over time. 

TL;DR electrons absorb and store light energy, and later release it.