How does a car’s differential work? You probably have a vague understanding. Some stuff spins, some stuff doesn’t, and somehow, the result allows your car to drive around a curve without shredding its tires or chewing up its own guts. There are gears and other gears, and it’s basically magic. That was the extent of my own understanding, until I found . It’s still the best, simplest, most immediately understandable explanation of how a differential works I’ve ever found.

I love this old black-and-white film so much, . It’s worth presenting again to a new audience, because even if you’ve held the innards of a disassembled diff in your hands, you might still struggle to explain how that collection of ring, pinion, and spider gears gets power to the ground, and lets two wheels on the same axle spin at different speeds.

Our instructor in this lesson is Henry Jamison “Jam” Handy, a fascinating character who spent much of his career making educational films like this. Most of the time, they were cleverly-disguised advertisements — most of Handy’s automotive films were done at the behest of General Motors, while others were sponsored by Standard Oil. Regardless of the topic, a Jam Handy film starts with a simple question of “how does X work,” and answers it with clear, clever, immediately understandable visual aids. Typically, the last minute of the film is where it becomes an advertisement — for example, hyping the latest technological advancement you’ll find at your friendly neighborhood Chevrolet dealer. But everything leading up to that brief sales pitch is general-interest, brand-agnostic knowledge that’s absolutely fascinating for car enthusiasts or anyone with a curious mind.

So! Back to the differentials. Naturally, this film from 1937 doesn’t delve into the voodoo of limited-slip differentials — we’re only talking about one-tire-fire open diffs here. But the visuals and the explanation are so simple, a kid could grasp it.

So take 10 minutes and learn the inner workings of something you always wondered about. Even if you already know the ins and outs of differentials, I promise this film will give you a new perspective on how it all works.