Take this with a grain of salt, because it was only explained in passing, but it was on Discovery, so if it was wrong I think they’d edit it out. Anyway…

Motion sickness, such as that caused by reading while driving, is caused because your eyes are telling your brain that you are not moving, while your ears are hearing the sounds of movement.***

The disconnect makes your brain go haywire, and it responds by sending you a queasy feeling.

Source: Time Warp on Discovery.

If you’ve never seen it, it’s the show where they use high speed cameras to capture slow motion video of interesting things. I usually don’t watch it because I don’t find the hosts likable, but last night they were in Las Vegas getting slow-mo video of Penn & Teller magic tricks and Cirque de Soleil performers.

Penn & Teller were amazing. They did a cup and ball trick with plastic see-through cups, and even in slow-mo they had to highlight the “trick” part of the trick before I could notice it.

Also, they had slow-mo video of chicks from The Fuel Girls getting set on fire.

And yes, that last part, while true, was a shameless ploy to generate traffic.

*** A more accurate description of the reason from the comments:
The inner ear is filled with fluid and lined with little hairs. As you move the fluid sloshes around and stimulates the hairs. The brain then interprets this to determine which direction the motion is in. This is why you cannot walk a straight line after spinning in a circle; the fluid in your inner ear is still spinning, telling your brain that you are still spinning (which can be solved by spinning the other direction).

So, the inner ear feels the forward motion of the car, the eyes see a motionless page of a book, and you end up puking your guts out on the side of some AZ highway that uses the metric system.