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!
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and I also have a diy podcast called mechanicalmashup.tv