First, I used oil, water, green food dye and toilet tablets.  

I used a small coke bottle.  

I poured in one quarter water and about three quarters oil.  

They soon began to separate. The water was at the bottom and the oil was on the top.  Then I added in the food dye to give it some colour. I used green dye.  

I then used a toilet tablet and popped it in. I had to split it in quarters because it was too big.  Then it started to bubble and then bubbles started to rise up to the top. This is because the toilet tablet is effervescent.  

The tablet makes the water full of air so it rises up but it falls back then when it gets to the top because it gets filled with water instead of air. The bubbles are then heavier and gravity makes them fall down.  

It fizzed for about fifteen minutes and then the bubbles all fell down and it stopped.