When I was small, I remembered having a toy that consisted of two glass plates, with coloured sand and water between them. When you tipped it up, the sand would slowly fall down (it would get temporarily trapped by air bubbles in the water) and you’d get interesting patterns forming as the sand fell.
Well, this has been bought up to date with a computer version, but it has evolved. Fallingsandgame.com contains versions of this game, where the sand is a continuous stream from the top of the screen and falls. You can put obstacles in its way, like plants, walls, etc and there will also be oil and water and salt falling too. Things get interesting as the elements interact – plants grow when they encounter water for example. There are many variants too – the pyro ones are quite spectacular!
Anyway, there is an on-line version you can play. Go and have a look. Hours of fun.
There must be a computer version of the original ‘two plates of glass’ game somewhere too, but I’ve not found one (although I’ve not really tried to look!).