Someone showed me this with an acorn a few years ago. Once you figure out how to do it well, you can use all sorts of objects to make a super loud whistle. This is handy if you are lost in a forest, just want to get someones attention, or if you want to ref an impromptu soccer game.
Caution as this is so loud, you may not want to try it indoors!
Please check out my other instructables and I also have a diy podcast called mechanicalmashup.tv
Step 1: The Hands
hold your hands so your thumbs are touching like the picture below. Rotate your hands open slightly like you are making a butterfly shadow puppet (see second picture) Note the "V" shape created at the top of your thumbs.
Step 2: Holding the Acorn Cap
You want to hold the acorn cap so it is totally sealed all around except for in the "V" shape you made with your thumbs.
Please excuse my hands, as an industrial mechanic, this is about as clean as they ever look.
Step 3: The Blow
Press your lower lip gently against your thumbs as shown, your upper lip kind of sits on top.
Blow hard. It will probably take a bit to figure it out right but when you get it, it will be loud!
Step 4: Video Examples