I know as kid I loved lava lamps and had a few spotted around my bedroom. I came across this video of an easy make-yourself lava lamp.
Love it when you turn lights down and pop the bottle on a torch!
This kind of demo would be great if you were doing a geology or volcano themed party.
Quick science info:
1) The oil is less dense than the water, so when you pour oil into the bottle of water the oil stays at the top
2) The food colouring is water based so it goes straight to the bottom and mixes with the water
3) The Alka Seltzer starts to fizz when it reaches the water – this fizzing is because of a realise of gas known as carbon dioxide.