When I was a kid we used to make these whistles out of jam tin lids.
Apart from being a fun toy for kids, I think these whistles would be great working with dogs, directing cranes, or even to attract attention in an emergency. They are pretty loud and you don't have stick your fingers in your mouth - this can be a pain if you happen to be wearing gloves or your hands are dirty.
I have heard that similar whistles are popular with shepherds in New Zealand who use them for directing their dogs.
Step 1: What you will need
1 lid from a tin can
A drill with a bit about 3mm
hammer and centre punch
A small piece of ply or something to bend the tin around. About 4mm thick.
A knife for cutting the duct tape.
Step 2: Make the tin safe
Step 3: Make your Whistle
Clean up any sharp bits from around the hole, and that's it.
Just look at the video to see how easy it is.