Basically your body is getting conflicting signals.
If you’re in a car or on a bus if you are looking at the floor 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 (normal enough) your body thinks it is being poisoned and the only way to deal with poison is to vomit it all up. Lovely…
Top tip, to avoid getting motion sickness sit at the front of the bus or car and focus on the horizon. Or suck on ginger, that calms your stomach down.
And here’s some related reading on ginger.
Related videos: more brains!