The motion sickness mystery
These days, the prevailing belief is that motion sickness arises from a disagreement between your eyes' visual input and your inner ear's sense of acceleration and/or movement. For instance, if you're in a plane that starts to bank sharply to the left, your inner ear tells you you're moving even though your eyes tell you you're clearly sitting still in your seat. The same holds true for reading a book in a car, and anyone who's ever gotten motion sickness in a movie theater knows the opposite can be just as uncomfortable.
It's not that simple, though (why would it be?): the scientific community still isn't 100% convinced that sensory disagreement alone is responsible for the motion sickness people experience in cars (or boats, or planes). Nor are they sure why nearly one third of us are more sensitive to it than others, or why women seem to get it more than men -- there simply isn't a consensus, and since nobody's funneling millions of dollars into motion sickness research, definitive answers don't seem close at hand.