I found this idea off of a few websites. It's such a fun and easy science experiment you can do with kids.
These are the supplies you will need: Water bottle, Vegetable oil, food coloring, and alka-seltzer tablets cut into 1/8ths.
Then, you fill the water bottle with a little bit of water on the bottom. I think I did about an inch. Then you add the Vegetable oil almost all the way full. Let it separate for a second.