What causes sea sickness or car sickness?

Motion sickness. One theory is that it functions as defense mechanism against neurotoxins. The area postrema in the brain is responsible for inducing vomiting when poisons are detected, & for resolving conflict between vision & balance. When feeling motion but not seeing it, the inner ear transmits to the brain that it senses motion, but the eyes tell the brain that everything is still, hence brain responds by vo.