Start by cutting a slit in the straw about a third of the way down. Do not cut all the way through.
Pour water into the glass.
Bend the straw 90° at the cut. Keeping the straw bent, put it into the water and blow.
Whistling, times two! Raise and lower the straw in the water to get a different pitch.
So what's happening? Your breath creates vibrations in the straw, which become waves that travel to your ears. The more air that is in the straw (in other words, the higher out of the water it is), the lower the pitch will be because the waves are longer. Yea, science! And yea, Little Passports!