Making a lava lamp / Fireworks in a bottle.
Small clear plastic bottles (500ml work best), baby oil, water, food colouring, alka seltzer, warning stickers.
Fill the empty plastic bottle 1/3 of the way with water. Add a few drops of food colouring and watch as it disperses. Float baby oil on top until the bottle is 2/3 full with liquids. Add an Alka seltzer to the bottle and secure the cap quickly. The alka seltzer causes the coloured water to fizz and rise to the top of the oil. The air escapes and the water droplet sinks back down simulating a lava lamp. Depending on the size of bottle it can last a few minutes. When the cap is removed it will restart briefly as the gases are released. If YP is going to take the bottle home with them please ensure you have a warning sticker to advise against drinking. These also look particularly good if you can dim the lights and shine a torch through the bottom of the bottle when adding the tablet.