When out in the woods alone, or if you get separated from a group, it helps to have an acorn cap available. If you need to call for help (or just attention), redditor prater77 explains a simple technique you can use to create a very loud whistle.
An acorn cap makes an extremely loud whistle when used properly. If you are looking at your two thumbs, place them together. Separate the tips of the thumbs to make a V. Hold the acorn cap in your hands with the top rim of it crossing the V you made with your thumbs. Put your top lip on your thumb nails and the bottom lip below the thumb knuckles. Blow. Adjust your hold until you get a high pitched, very loud whistle. When fishing at night on rocky piers, I carry a few acorn caps in my pockets and fishing bag just in case I slip, fall, and get hurt.
For a picture-based tutorial, check out WikiHow.
If lost in the woods, don't shout for help. Look for an acorn cap. | Reddit
Photo by Dirt Time.