Hold one straw vertically in the water and the other horizontally
so that the ends are close together. By blowing air into the
horizontal straw, one can lift the water up
and spray it out of the vertical straw. This usually takes a little
moving around of the blowing straw.
The air from your breath curves around the top of the vertical straw.
The streamline curvature allows the flow to sustain a pressure
difference between ambient and at the straw tip. The harder
you blow, the faster the flow or the lower the pressure and therefore the greater the force to pull
the flow up the straw. Note that the air from your breath will not enter the straw, and it usually takes
some time to find the correct placement of the horizontal straw to get the best interaction between the two straws.