Bernoulli's Principle (spelling)As the velocity of a fluid increases, the pressure decreases. If you push water through a pipe, have a choke on the pipe (narrow section), then open it again, you'll find different pressures at all 3 regions (before, during, after... i don't remember which is which right now... i haven't had coffee).
This is the same principle that allows flight. The air (a fluid) moving over the wing moves faster to cover a longer distance, so it's pressure is lower. That sucks the plane up like there's vaccum cleaners above the wings.
In the shower, the water falling down (with the vortexes in the air created by the drops) creates a low pressure region, because the air outside the curtain is not moving. This sucks the curtain in like an aircraft wing.
Hope this helps... all from memory 
That engineering degree was good for something!
