On a corner, a car’s outside wheels have farther to go than the inside ones, so they have to turn faster. The gearbox in the middle of the axle that allows this is the differential; the version in cars is usually credited to Onésiphore Pecqueur, a French engineer, in 1827. It is fair-minded to a fault: it always pushes the two wheels with the same force, and lets them sort out their speeds between them. On a dry road that’s perfect. Put one wheel on ice and the fairness becomes the problem. The ice can only take a tiny push, and since the two wheels must get equal shares, the wheel on good ground gets the same tiny push, and the car sits still while the icy wheel spins away. All the motion goes to the one that is getting nowhere. Engineers had to build a second mechanism, the limited-slip differential, to tell the first one that equal isn’t always the same as fair.