This is a beautiful transparent acrylic model of the differential found in any car that has a single engine (even single-engine electric cars). If you want to drive two wheels with just one engine, you have to content with the fact that those wheels will need to turn at slightly different speeds when the car is going around a corner. The wheel on the outside of the curve has to travel farther, and thus must turn faster than the wheel on the inside. The differential is what makes this possible.
This model lets you see how the differential uses a clever arrangement of four gears to pull off this clever trick. Because all parts of the model are clear, you can see exactly what's happening everywhere inside.