Three marbles let go at the same moment from three different heights. Every track is a piece of the same curve, a cycloid, the curve a point on a rolling wheel's rim draws, cut off at a different height. The one near the top falls almost straight down; the one near the bottom barely moves. And still all three reach the bottom at the same moment, every time. Higher up the curve is steeper, so a marble that starts higher falls faster, and the extra speed exactly pays for the extra way. A switch sends the marbles to a different track every race, and the tin photographer's flash fires once for all three: a photo finish, and always a tie. But his tripod's front leg is creeping on the polished shelf.

Every knock and every music-box note is timed by the machine itself.

Stay for the end.

The curve is Christiaan Huygens's tautochrone, 1659.

Which one would you have bet on?