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).
Step 2: Drilling and ripping
Drill a hole in one of the caps, somewhat off-centre.
Inside the cap, rip out the lining of the cap from around the hole - this makes the edge of the hole thinner, and better at splitting the air.
Step 3: Gluing
I used hot-glue, because the two caps had slightly wonky edges. The hot-glue filled in the gaps nicely.
Step 4: Adding the straw
When you have the best position figured out, tack the straw in place with a couple of drops of hot-glue, check again that the position is correct, then add more glue around the straw to hold it in place.
I also trailed a couple of strings of glue over the straw, with the idea that they would keep the straw in the correct oval shape. I don't know if they are actually needed, but they didn't damage anything...
Anyhoo, there you go - the whistle has a pleasant tone, a bit like a small train-whistle, and is not so loud that small children will run screaming from the room, or get in major trouble for playing with it in the house.