Step 1: Fill your 1L water bottle up with water so that it will be 1 part water 2 parts cooking oil.
Step 2: Pour in the 2 parts oil as stated above (oil will stay on top because water is more dense). Two experiments in one!
Step 3: Pour in a teaspoon worth of food colouring.
Step 4: Now just drop in an alka-seltzer and the magic will begin! Don't leave the cap on. (When the alka-seltzer dissolves away, drop another one in.) Little tip here, when it's dark put a flashlight just under the bottle, to get that true glowing effect!
Why does this happen? So it all started back in step 2 when the oil didn't mix with the water and this is because water is more dense then oil. When you dropped that Alka-Seltzer it starts to fizz, producing bubbles of carbon dioxide that will float up and bring some coloured water for the ride. When the carbon dioxide reaches the top and releases the gas, so back down the coloured water goes. There is the science behind our lava lamp!