Today I am going to teach you how to make a whistle from a bottle cap. This could be very helpful if you were to get lost because whistling takes a lot less effort than shouting. You can also do this on an acorn and if you're really good on a bottle!
For the formation you need to hold the cap in your hands and use your thumbs to make a "V". Do not put your thumbs flat on the cap. Make sure you make a small gap instead of a large gap or the air won't flow correctly.