In one of literature's most famous quotes, Hamlet (the star of the play by the same name), apropos of his supposed insanity, declares "I am but mad north-northeast; when the wind is southerly I know a hawk from a handsaw." (Meaning: He is only insane when the wind blows from North --> South, but sane when it blows the opposite direction) The reason for this puzzling metaphor is that "handsaw" was a misprint for "hernshaw," a young heron. If you could tell a hawk from a heron (its prey), you evidently had at least some of your wits (sanity) about you.
Equally understandable is the more recent explanation, that "handsaw" meant saw, and "hawk" meant the square board, with a handle on the back, on which a mason holds the mortar on as he spreads it on the wall. Naturally, anyone could tell this hawk from the carpenter's tool as easily as a hawk from a heron. Since it makes sense, and makes him sound sane either way, take your pick.
Sources: AP English Notes. Node your homework!