I use this experiment with my KS1 students to explain the basic principles of experimentation. It's very simple to set up and most homes have all the necessary materials.
It's also useful at KS2 and higher levels to explore and explain the physics of density.
What to Get
Small objects (to put in the oil/water mixture)
What to Do
Get a clear container. If you have a lot of choice, taller is better.
Fill the container about 1/3 of the way with water.
Explain the idea of density: tailor this to your student's level. Include the basic relationship of mass and volume (Density=Mass/Volume). Ask your student to explain what this means, "How dense will an object with a big volume and very little mass be?".
Now explain that objects with density which is greater than the density of water will sink, and vice versa.
As an example, pour the oil into the water and see what happens. Ask your student to compare the density of oil to the density of water.
It should look like this:
After discussing the different density of water and oil, and identifying that oil is less dense, make guesses about the density of your collected objects.
This is a good time to discuss the importance of predictions/hypotheses in experimentation.
Once you've recorded your predictions, it's time to test!
Drop the objects into the mixture and see where they land.
You can use different types of liquid to create more layers and test a wider range of densities.
It should look something like this*:
Use food colouring to clearly distinguish the different liquids.