jfruh writes: "People who like watching American football on TV have come to take a great technological development for granted: they can see on TV a yellow line that indicates how far the offensive team needs to move the ball to get a first down. The line, which has been used on most football broadcasts for more than a decade, is actually something of a technical marvel, with a lot of work going into creating something that's not flashy or obtrusive but enhances the game-watching experience. Even those bored by sports can appreciate the science behind it." Link to Original Source
We warn the reader in advance that the proof presented here depends on a
clever but highly unmotivated trick.
-- Howard Anton, "Elementary Linear Algebra"