We've all heard the amazing loud "survival whistles" made from bottle-tops or acorn caps.
I wanted to make something more ... controlled.
(you can probably work out what to do just by looking at the photo, but read on.)
Step 1: What you need
I made this with two soda bottle caps and a piece of drinking straw, using hot-glue, my Leatherman and a drill with a twist-bit for wood (I'll warn you in advance that the hole in the caps must be drilled - if you melt the hole, the edges will be too rounded to make the whistling noise you need).