Courtesy of wikipedia.org

The behavior of water and air can depend not only on factors like pressure and weight, but also motion. This nifty demonstration recycles used plastic bottles to show basic physics concepts in an exciting and interesting way.

Here's what you'll need:

Two plastic water bottles
Waterproof tape
One washer (that fits the bottle-lids snugly)
Stopwatch

## Step 1: Start Recyling!

First, grab a pair (or more!) of clear empty plastic bottles of the same size.

## Step 2: Half-Full

Fill one of the bottles with water.

With your stopwatch ready, pour out the full bottle. Record how long it takes to drain with its "glug glug" method.

## Step 4: Get Crafty

Place the washer on top of the refilled bottle. Hold the empty bottle on top of the washer and tape the setup together securely (this section has to be water-tight, this is where the vortex will be formed)!

## Step 5: Swirl of Science

Flip the filled bottle so it is on top, give the setup a good swirl, and watch the water whirl!

It may take a few tries to get the hang of how much to swirl the water; don't be discouraged.

## Step 6: Learning Why

Gravity is pulling the water down, but giving the water that initial swirl starts a spiralling drainage pattern, the same effect seen when cold air falls in a swirl as warm air rises (as in a tornado!).

The reason this drains faster is because, if you look close, you can see that throughout the whole process there is a hole where the air can flow up, surrounded by draining water. This makes for a much faster drainage than the traditional glug-glug of pouring from bottles. The glug-glug comes from the water and air having to both use the same passage; when the air is rising, the water ceases flowing and vice versa. This makes for slower drainage than the continuous vortex.
Cool idea, I need to try that with my girls. The vortex in the bathtub always gets many ahs and ohs. I wonder, would it not be possible to assemble the two bottles as you described and get the glug-glug just the same if you do not spin? I think it would be easier to demonstrate the difference... Then again, the water may stay in the top bottle if you turn it too quickly, but that would be something to explain for the kids, too.
Absolutely! In fact, it takes some finesse to get the vortex action described, and the first attempt generally does feature the glug-glug drainage characteristic of normal pours. <br> <br>If you spin too quickly you can keep the liquid in the top bottle through centrifugal force, for sure! Thanks for your interest and input, I hope you all have fun with it :)