GoKids

You will need

• 2 1L soft drink bottles without lids
• A washer with centre hole diameter of 9mm
• Electrical or plumbers' tape
• Food colouring

What to do

1. Fill one bottle around 2/3 full of water and add food colouring. We’ve done this step for you.
2. Place the washer over the bottle’s mouth. Place the empty bottle upside-down on top of the washer and tape the two bottles thoroughly and firmly together with the washer in the middle. We’ve done this step for you.  
3. Turn the Vortex Valve upside-down so the water filled bottle is on top; note how the water flows and how quickly.
4. Now, smoothly rotate the bottle to start a vortex. The easiest way is to hold the bottom bottle as you rotate the top bottle in a smooth, steady arc. Does the water flow differently with a vortex?

What's happening?

As the water forms a vortex and drains into the empty bottle, air rises up through the middle of the vortex into the top bottle. Where have you seen a vortex before?

There are several forces involved: gravity drains the water, centripetal force causes the water in the vortex to be pulled to the centre, and air pressure pushes water from the bottom bottle into the top. The water moves faster and faster as it gets closer to the centre of the vortex.