Question: how does glow in the dark work? And why is it always ''greeny'' coloured?

Tom Hardy answered on 15 Jun 2010:

There are lots of things that can cause the effect you know as glow in the dark. These include bioluminescence (living organisms use reactions that convert chemical energy to light energy), chemiluminescence (a chemical reaction that gives off light without heat), radioluminescence (light produced by radiation – this is the one you see at the start of the Simpson’s).

Something called phosphorescence causes the light in most glow in the dark items. Phosphorescence takes place then an object takes in energy (absorbes it like a spongue does water) but then releases this energy as light very slowly, because it is released slowly you can see it evenafter the light is removed. Phosphorescence is actually blue but scientists have realised that mixing chemicals with the phosphorescent material can make other colours. Chemicals called zinc sulfide and with copper are often mixed with sodium fluorescein which is a green dye. This makes the yellow-green colour you mention. But you can also get pinks, reds, blue and orange dyes.