"<red>Misidentifying a knocking sound means this" they would not mistake a sound very similar to a knocking sound for a real knocking sound. Hitting a pillar to make something similar to a knocking sound is no good. When you record a knocking sound on a cassette tape and play it back, it becomes `the sound of the tape with a knocking sound on it', and not a knocking sound. So that's no good either!<white>"