Glass marbles fall from brass trays onto a sorting wheel. Each one rolls until it passes a jar of its own colour, and drops in: three marbles, one jar.

The wheel is the whole puzzle. Jars fill and lift off, a new colour slides into the queue, and a colour whose jars run out before its marbles do leaves one marble rolling with nowhere to go.

When a marble rides a full lap the machine jams and the level ends. So the question is never speed — it is order: which colour to feed, and when.