For this simple experiment we recommend using a tornado tube but you can also make a version using a washer and duct tape.
Fill a 2 liter soda bottle 3/4 full of water. (We also added some glitter for fun!) Then attach the tornado tube and another 2 liter bottle to the other end of the tornado tube. Now flip the bottle with water upside down, give it a swirl and watch the vortex form as the water moves quickly into the other bottle. So much fun and my boys have played with this setup so much since we put it together!