Use density for your own amusement! You will need:

Transparent Jar with tight-fitting lid (jam jar or similar)

Water

Vegetable Oil

Food Colouring

Salt

Torch

Glitter (optional)

What to do:

Fill the jar to around the 3/4 mark with water. Then add food colouring of your choice (a drop at a time) until you have your desired colour. Add glitter Fill your jar almost to the top with oil and leave for 15mins to settle Add teaspoons of salt - you will start to see a lava-lamp effect. Shine a torch through the jar and the effect is complete! NB When the salt dissolves, the oil will return to the surface. Add more salt to see the effect again.

What's Happening? Oil is less dense than water, and so rests on top. When you add salt, it sticks to the oil; because salt is more dense than water, it sinks - when enough salt attaches to a blob of oil, the whole blob sinks. When the salt dissolves in the water, the oil's low density causes it to rise up again.

Next? Experiment with different ratios of oil: water, different types of oil, still vs. sparkling water.