- a bottle (I used a whiskey bottle, simply because it was there)
- vegetable oil
- food colouring
Fill the bottle about a third with vegetable oil. Add food colouring, then water to the near top. Wait for oil and water to separate. Hand to small child for experimenting with making waves, and discovering that oil and water are two liquids that won't mix (tip: gentle slanting will make prettier waves than random shaking).