system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is actually happening when, after a day in the ocean or riding rollercoasters, the body will almost simulate the feelings when you're staying still?
gpt
There's a few reasons why this happens, and it's actually been a source of contention for centuries now. This is part of where the term "Sea Legs" comes from.

When you are on a boat, or a roller coaster or other similar behaviours, your brain, vestibulocochlear system (The tubes of fluid in your ears that help maintain balance) and muscles all grow accustomed to that environment.

What you're actually experiencing after coming back from such things is actually almost the opposite of what you were feeling; your body is so used to that swaying motion of a boat that the complete steadiness of dry land temporarily "Confuses" it.

Have you ever been on a trampoline for a moderate length of time? That's a very similar scenario; after coming off the trampoline your legs feel heavy and your muscles seem weak because you've become used to the "Springy" terrain.

It's a similar situation here; You're used to the movement, and in its absence your brain is preemptively simulating movement that is no longer there, making you feel like you're moving even when you're sitting still.