I don't get motion sick, and now I realize the concept of being on a boat and feeling as if the ocean is moving back and forth is completely alien to me. When I'm on a boat, the horizon appears perfectly still to me. If the boat tips, the boat looks like it's tipping to me. Likewise, if I stand on my head, the world looks right side up, but I feel as if I'm upside down. In movies sometimes they'll place a camera upside down to indicate a character is doing sit-ups on a hanging bar or something and I always thought that was a silly way to portray that since the world doesn't look upside down when you are.