Why do we throw up when we get motion sickness?

Motion sickness: One theory is that it is a defense mechanism against neurotoxins. When we feel motion but can't see it, the inner ear transmits to the brain that it senses motion, but the eyes tell the brain that everything is still. As a result the brain concludes that one of them is hallucinating & that the hallucination is due to poison ingestion. The brain responds by inducing vomiting to clear the toxin.