What you need: water bottle, oil, water, food coloring (the liquid kind not gel), and Alka Seltzer tablets.
How to do it: Fill the water bottle about 1/3 of the way up with water and add food coloring. Then fill the bottle the rest of the way with oil, pouring slowly to show how the two separate. Leave a little room for air at the top of the bottle. Once the oil and water have separated, split the Alka Seltzer tablet in half or thirds and drop it in the bottle. The tablet will fizz and colored bubbles will rise through the oil. Awesome!!!