We don't completely understand how seasickness is caused, or why our bodies do it, but there are some interesting theories. It's thought to be brought about because our body feels that it is moving, but our eyes can't tell very well that we're actually moving. This is because although everything next to us in the boat is clearly at motion (the boat is moving around in the ocean), because we are in the boat and everything else is also inside the boat, our eyes look around and don't see anything moving differently from us. This is especially true when you're in the middle of a large body of water and have nothing outside of the boat to tell you that you are actually moving. So, this confuses our minds -- we can feel that we're moving, but our eyes don't think we're moving, and this causes us to vomit. One current theory for why this makes us vomit is that our bodies think that we have ingested a toxin that has made us hallucinate -- why else would we feel like we're moving, but it doesn't look like we're moving? So our body tries to protect us by vomiting, to get rid of the toxin. Also, moving in a boat puts irregular pressure on our bowels and diaphragm, which probably contributes to the nausea. For more information check out these websites:

Seasickness is not a real illness, but a form of motion sickness. Basically, the motion of a boat (rocking and swaying) disturbs the organs of balance in your inner ear. This results in nausea and dizziness.

You have organs in your ears that detect direction, and if those organs get jostled in the wrong way or that you're not expecting, it can make you feel sick. I'm not sure it's known how this happens.

The short answer to your question is that when the messages from your eyes and the messages from your inner ear dont match up, you feel sick.

You know that we hear with our ears, but the inner ear also has sensors that tell our brain our orientation (standing up, leaning over, lying down, etc.) and our movement. The semi-circular canals do this job. They are filled with fluid. When the fluid moves, it pushes on sensory hairs. These hairs send messages to our brain. Take a clear bottle of water and move it around. You can see the general idea.

When were on a boat, our inner ear tells our brain that were bobbing up and down with the waves. But if were looking at the inside of the boat, it looks still. If were up on deck, the horizon is still.

No one knows exactly why our brain responds to the two different messages by making us feel sick, but maybe its because when our senses are acting strange, we may have consumed something poisonous, and vomiting is how our body gets rid of the bad stuff.

Astronauts sometimes have a problem like seasickness. Can you figure out why?