Materials - Water, Oil and Alka Seltzer (effervescent tablets/ look for ENO tablets as an alternative)
Suggestion - if you do not effervescent tablets handy, you can use a few spoons of regular salt - the reaction maybe slower but it will still work.
In a tall glass or jar, add 2/3 water and add a few drops of food coloring.
Pour oil on the top, leave some space on the top of the glass for the bubbles to fizz so it does not drip out.
Now, add the Alka Seltzer tablet, the effect will continue for some time, when it settles, repeat.