This is a wonderful experiment to explain the concept of liquid density and gases evolving from the liquid. Start by explaining that water is heavy so it settles down and oil is lighter and floats on top of water.
Step 1: Take little less than half glass of water and add some food color.
Step 2: Now add equal amount of cooking oil to water. Because of the difference in density of water and oil, both will get separated.
Step 3: Now put some baking soda/antacid tablet
and enjoy water and oil mixing bubbles.