This is a simple Instructable on how to make a straw whistle.
Step 1: Cut the Straw
Cut the straw however long you want. It should be at least 4 inches though. The longer the straw, the lower pitched the sound is.
Step 2: Cut Again
Next, cut edges off about 1 cm down from the tip of the straw. Cut the edges at about a 45 degree angle.
Step 3: Hold It Down
Hold the end of the straw with your fingers really tightly until the tips are about 2 mm apart. This may take a while.
Step 4: BLOW!
Blow into the end that you held down now. It will make a funny noise!