system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the propensity for motion sickness increase as you get older; e.g., riding roller coasters or other rides at amusement parks?
gpt
Balance is controlled by a group of small organs in the ears, called the vestibular system. These organs detect two different kinds of motion: rotation and movement in a straight line.

The way straight line movement is detected is that there are tiny little bones shaped like bits of stone that sit in a kind of sac that is lined with tiny hairs. When you move in a straight line, the little stones press against the hairs, which send signals to the brain that say “hey, we’re moving forward!” or “hey, we’re tipping over!”

The way rotation is detected is with three rings inside your ears.  These rings are filled with fluid and more tiny hairs, and they are arranged in a way so that no matter which direction you move, the fluid presses on the hairs, too. So, if you did a cartwheel, one of the rings would say to your brain, “hey, we’re doing a cartwheel!” If you spin around like a ballerina a doing a pirouette, a different ring would tell your brain, “hey, we’re doing a pirouette!” And if you nod your head up and down, a third ring tells your brain, “hey, our head is nodding!”

The fluid in our ears is roughly the same as blood.  Sometimes, that fluid thins out, which makes movements seem more intense than they actually are, which can cause problems with balance. Sometimes the little hairs in the organs will break away,  and the spot where they were either sends too many signals to the brain, or none at all.  This can also affect balance.