By Tim Johnson
Take two empty 2L pop bottles (labels removed). Fill one ²/³ full of water and place a washer on its mouth. Invert the other bottle onto the washer and duct-tape the two bottles together (firmly, using glue if necessary) at the connection. Turn the bottles over, so the full one is on top, and swirl a few times—then watch the vortex form as the water drains from one bottle to the other. Add food colouring or small pieces of foam to observe more clearly.