It's become an important part of our EDC (Every Day Carry) over recent years: the whistle. The classic use of the safety whistle is, of course, signaling you're in distress and need help: three sharp blasts in succession, followed by a pause of at least one minute, and repeated until a response is received. The shrill sound of a whistle is likely to carry much further than the human voice and requires much less energy. Remember, the number three is key in distress situations: a distress signal can be three fires or piles of rocks in a triangle, three blasts...
