The eyes tell the brain "we are moving." The inner ear tells the brain "nope we do not move at all". The muscles tell the brain "We try to make us moving." Something undefinable tells the brain "In fact, everythign moves around us" .CPU cannot compute all this conflicting data too well. If then the eyes push it over the top and tell the brain "we are not moving as fast as the msclues try to move us", then something goes perplex over this latency issue, and makes a desperate clicking sound of resignation, and the machinist in the brain presses the red stop button: we feel nausea.
Like on Monday mornings, first hour of the new week's shift. Your conscience tells you that life shall be good. Your senses tell you by all they perceive around you: it isn't good at all. The boss walks in and says this sad state will last for another full week before it will become good on weekend. You vomit on his table.
See...? LOL