It seems like magic. In an attempt to make it audible, you feed a faint signal, a wave of a particular frequency, through an amplifier—a strange black box that you bought at a junk yard, because your proposal to buy a proper one was turned down once again. Of course you hear nothing. The darn thing doesn't seem to work. Out of sheer destructive frustration, you add some random noise to your signal, and voila! Suddenly you hear it. The noise, rather than blurring your signal, makes it come through loud and clear. Take the noise away and the signal vanishes again.