When the rollercoaster plunges, your insides lift upward while the rest of you rushes downward, and as the motion continues, you begin to feel as if your organs shift out of position until you eventually reach a halt.
You see, your intestines are loosely supported within your body, so when you start to drop, your organs trail behind since they are not firmly anchored.
This creates a sudden near weightlessness, which causes the nerves around them to fire signals up to your brain, giving you the