Algebra, without which we could not quantify all the nice engineering stuff, or figure out many of the Car Guys' puzzlers. ("You can only do it if the answer is 'two.'")

And we absolutely cannot leave out the bicycle, which introduced rubber tires and chain drive which allowed automobiles, motorcycles, and airplanes to come into being.

And the bicycle of the late 70s is the best of all human engineering, period. A person on a bicycle uses less energy per distance than any other machine, or human or animal on Earth.

Saul