Car sickness is essentially the same as sea sickness or air sickness - it all comes under the banner of motion sickness.
Basically your brain is receiving conflicting signals from your eyes and ears. If you are looking at the floor when in a car or on a bus or a book your eyes are telling you that you are not moving. But your ear is telling your brain that you are indeed moving. So instead of your body realising you are reading a book on a bus, your body thinks it is being poisoned and the only way to deal with poison is to vomit it all up.
One way of getting around this is to look out of the window into the distance. It will mean that the signals from your eyes and ears match.
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