The theory we like best is that it’s your body’s reaction to perceptual dissonance.
While riding in a car, you’re in an enclosed area in which your eyes perceive that nothing nearby is moving, but your ears and the rest of your body feel like you indeed are moving.
In an effort to solve that enigma, your mind starts your body puking, perhaps interpreting the inexplicable dizziness as an onslaught of poisoning. (Compare, for example, the body’s reaction to a first experiment with tobacco or too much alcohol.)
Given that, the non-medical cure for motion sickness involves making sure your eyes are in sync with your ears by looking out the window toward the horizon.
Above all, avoid anything (reading, for example) that keeps your eyes focused inside the passenger compartment.
Yes, the whole theory sounds a little stupid, but the cure seems to work for most people, as evidenced by the fact that while car passengers often get motion sickness, car drivers almost never do.