This week's experiment is one that has been on my list of possible experiments for quite a while. The idea is simple and lots of fun.

You will need:

- 2 plastic, two liter soft drink bottles
- water
- a bathtub or large sink
- a friend

You may want to try this several times, so in order to save water, wait until you are going to take a bath. That way, you can make good use of the water, instead of wasting it. Fill both bottles with water. You take one and give the other to your friend. The goal is that when you say go, both of you will try to pour the water out of your bottle. Whoever gets all of the water out of their bottle first wins. You are not allowed to squeeze the bottle to force the water out faster.

Now for the trick. To win the race, as soon as you turn the bottle upside down, give it a quick swirl to get the water going round and round in the bottle. While the water is slowly "glug, glug, glugging" out of your friends bottle, it will flow quickly and smoothly out of yours. If you practice a few times first, you should be able to swirl the water without making it obvious to your friend how you won.

Why does swirling the bottle make a difference? Try it again and watch closely to see if you can figure it out. Be sure to try things both ways, with the swirl and without. Observe carefully both times.

Did you see the difference? For the water to go out of the bottle, air must go in. If you just turn the bottle upside down, then a little water comes out and then it stops while a bubble of air goes in. Things alternate between water coming out and air going in. This slows down the process of getting the water out of the bottle.

When you swirl the water, you form a vortex, which looks like a miniature tornado. This vortex forms a tube-like pathway which lets air come in while the water flows out. You get a constant flow of air inwards and water outwards, draining the bottle at a much faster rate. You can see the same thing happening when you pull the plug in the bathtub. As the water flows down the drain, it swirls and forms a vortex. As soon as this vortex forms, the water drains much faster. With the bathtub, the vortex is letting the air out, so the water can go in faster.