This is a great investigation or experiment for making predictions, states of matter, and so much more! For a investigation, follow the instructions below. For an experiment add a question: Does the type of food oil matter (try vegetable, olive, canola, etc.)? Would sugar or sand produce the same effect as salt?
Procedure: Fill a clear plastic cup about 2/3 full of water, add 6-8 drops of red food coloring, slowly pour the oil into the cup (at this point, the oil will separate and float on top), sprinkle salt on top of the oil, and watch blobs of lava move up and down in your cup! Your own lava lamp! :)