NIGHT BRIDGE

The rabbit crosses the bridge in one minute, the fox in two, the bear in seven, the turtle in ten. The bridge holds two at a time, a pair moves at the slower one’s pace, and in the dark nobody crosses without the firefly jar — so someone always has to bring it back. How fast can all four get across?

• The fastest of the group carries the jar, each in its own way: the rabbit in its mouth, the fox from its tail, the bear in its paw, the turtle on its shell. When the rabbit crosses with the turtle, it hops onto the turtle’s back.
• The planks light up only as the jar comes near. The firefly circles the jar, stars twinkle, the river flows, crickets chirp, and every animal has a footstep sound of its own.
• Every trip is written into a log at the top: who crossed, and who brought the light back. The game also keeps your best time for as long as it stays switched on.
• The game never tells you the answer. Once everyone is across, it only says whether a faster way exists.
• The puzzle is known as bridge and torch, and the earliest known version dates from 1981. If two cross over and one comes back each time, there are exactly 108 different ways to do it.