Briefly: Like the harmonica, the slide whistle is one instrument you need no skill to play -- you just blow in one end, and slide the other in and out. Unfortunately, that means that handing out a bunch of these adorable Wet My Whistle musical straws at your next get-together will result in a room full of amateur musicians trying (and failing) to play a tune.
Sliding the whistle in and out of a beverage changes the amount of empty space inside, which in turn produces different notes when played. And in case you were wondering, yes, they do work as a regular straw, as long as you cover that air hole with your finger as you suck. [Fred & Friends]