On the weekend I did something I had not done for about 16 years. I went to the supermarket and bought a packet of disposable nappies (diapers). Why? It was all for science outreach!
This week I am doing a few demos at a kids club run by our church. Last year I did mentos and coke rockets. My glamorous assistant, my darling wife, found this video, which inspired us to increase our repertoire.
The key material is sodium polyacrylate which is a superabsorbent polymer. It is amazing that they can increase their volume by a factor of as much as 500. This is a phase transition analogous to a liquid-gas transition.
A nice follow up is to add salt which destroys the cross-linking and reduces the polymers ability to absorb water.
This is shown in the video below.
A nice mean-field theory of the salt induced volume-collapse is given in this PRL from 1980.