A survival whistle is a must in any good bug-out bag, but if you don't have one, or you're stuck without a way to signal to others where you are, a soup can lid will do the trick. This video from Specific Love Creations shows you how to make one, even if you need to open a can to do it.
Of course, if you're wandering through the bush, a can of soup may not be terribly common, but you may find an empty can somewhere with one side still on that you can use for this. If you have to open a full can, the video shows you how to do that with a flat surface, like a rock or concrete, easily. Once you have the can lid off, bend it in half, not all the way, and be careful since the edges will likely be sharp.
Puncture a hole in both sides with a nail or any other sharp object you happen to find, pry the lid open about 30 degrees and, of course, watch out for the side the nail pushed through, since that's the side that needs to face up when you put it into your mouth. Pop the now-bent lid into your mouth, make a nice seal around your lips with both holes so all the air flows through those holes, and blow. The video above shows you exactly how it's done, and how it will sound when you're finished — along with some safety precautions to avoid cutting yourself with this method too.
How to make a Survival Whistle from a Metal Can [Specific Love Creations]