What you Need
What to do
Cut the straw into two halves.
Hold one of half-straw in your mouth.
Put the other half into the water, and hold, or tape it so that it sits vertically, with about 1-2cm emerging from the surface.
Use the straw in your mouth to blow quite hard, across the top of the other straw.
If nothing happens, try lowering the straw in the water.
What may happen
You should find that when you blow hard enough, you produce a spray of tiny water droplets, just like an airbrush.
If you put some colour or paint into the waterm you can use it to paint with (though this could get very messy, very quickly, so make sure you do it outside).
Why does it happen?
Rapidly moving air will be at a lower pressure than when it is stopped. This is called Bernoulli's principle. So when you blow across the top of the straw, the rapidly moving air is at a lower pressure than the air above the water in your glass.
If you blow hard enough, the air above the glass will push the water all the way up the straw, and out of the top, where the rapidly moving air breaks the water up into tiny droplets, which are good for painting with.
Commercial airbrushes work exactly the same way, they just use higher pressures and better designed nozzles and produce smaller droplets.
This is also how fuel is mixed with air in an engine's carburettor: Air moves across a nozzle, and fuel is sucked up, and forms a fine, flammable mist, which is then burnt in the engine.
Why is fast moving air at a lower pressure?
Air, being a gas, can only be accelerated by a change in pressure. If the air is slowing down, then it must be going from an area of low pressure to a higher pressure, so the rapidly moving air must be at a lower pressure than the slower moving air.
This would make a great science and art project for kids.. combine two in one... Very cool Dave and I love the text boxes opening up at the straw so it refreshes the text and reiterates whats happening.. very nice set up Dave! Karen W., Sat, 5th Dec 2009
It's easier if you don't cut fully through the straw... John, Mon, 7th Dec 2009
Here's a really good application to Bernoulli's theorem.Good work. Stuti, Sun, 24th Jan 2010
The fast moving air in the straw certainly has a lower pressure than the slower moving air in the mouth and lungs of the kitchen scientist, but once the air exits the straw it immediately drops to ambient pressure. Moving free streams of air don't have a lower than ambient pressure. Using the Bernoulli effect to explain air brushes, perfume sprayers, and flit guns as well as movement of empty pop cans, ping pong balls, various pieces of paper when subjected to a free air stream, as a demonstration of the Bernoulli effect is widespread on the internet, but they are all wrong. This is a science site and should get it correct.
Hi Another article from the website mentioned by the author above as proof seems pretty adamant that this is not the Bernoulli effect: http://www.physics.umd.edu/lecdem/services/demos/demosf5/f5-10.htm drifter, Thu, 17th Mar 2011