This is a great little trick that you can easily whip out for comic relief next time your are sitting at a restaurant. The sounds produced range from a duck call to a flagulent.
This is a great sound production platform and yields well to modification. The possibilities are almost endless.
Step 1: Cut the Straw
If you prefer, you can see this entire Instructable in video format.
Flatten an inch or so of the end of the straw and then cut the corners of the straw off starting about 1/2 inch from the end. The remaining flaps are the reeds that produce the sound.
The sound produced is affected by the shape and length of the reeds, so try cutting the corners off at different angles and starting at different lengths from the end of the straw.
Step 2: Blow
Playing the horn is pretty easy. Just blow. If it doesn't work, try varying the volume of air you are blowing or biting down on the straw to flatten it a little more. I find that I get the best results when the straw lightly touches my top teeth.
Step 3: Modify It
In the video I show some basic modifications using a funnel and some PVC pipe. The sound that is produced is much fuller and louder than the bare straw.
I also experimented with fitting one PCV pipe into another to make something similar to a trombone. If you watched the video, I am sure that you were very impressed with my rendition of the Star Spangled Banner that I played on this instrument.