If someone tries to read while riding in a moving car or train, they may get an upset stomach. The eyes focus on the page and tell the brain that nothing is moving. But the inner ear feels the car moving and sends that message to the brain. When the brain gets these messages, it gets confused and sends its own message to the body to slow things down.
While the brain is deciding which message is correct, some people get an upset stomach or headache. Sometimes they start sweating or even throw up.