Reading a book in the back of a car can make you feel tired, because your eyes and ears are now "arguing" intensely, and the brain has to find a way to solve the problem.
When you are reading a book in the back of a car, your eyes see the book in a stationary state. So it will "tell" to the brain that you are standing still.
But your ears feel that the car is running . So it will "tell" the brain that you are moving.
The ears are not only for listening, they also help you balance.
The ear consists of 3 main parts:
The inner ear contains hairs that grow from above. Scientists are "hair cells".
Some hair cells help us hear. When sound hits the hair cells, the hair moves and the cells send signals to the brain. Our brain will use these signals to listen.
Other hair cells help us balance. When the vehicle you're sitting in moves, the movement of the car will move the hair on the hair cells, and then they will send a signal to the brain. Our brains use those different signals to know we're moving.
Some people's brains don't like the situation when their eyes say they're standing still, but their ears say they're moving.
When the eyes and ears argue like this, the brain can speculate that something dangerous is about to happen.
If this happens, the brain can put the body into a state of readiness for fighting or fleeing (scientists call this a "hit or run" reaction).
One of the things the brain can do now is take blood out of the stomach and transfer it to the muscles.
Giving blood to muscles can help us fight or run away. But when the stomach loses blood, we get the feeling of motion sickness.
If reading a book in the back of a car makes you feel sick, you need to "settle" the argument between eyes and ears.
One way to do this is to stop reading and look out the car window. That can help your eyes tell your brain you're moving, because you see the outside world passing by, and your ears tell your brain you're moving because you feel the car is moving.
But this way is not suitable for everyone. Some people still get drunk when they are in the car, even when they are not reading books at all.
That's because while the eyes and ears help us balance, our skin and muscles do the same thing. And yet there are many more controversies that force the brain to solve!