The "fluttery" or sinking feeling in your stomach during takeoff or turbulence is often caused by a combination of G-forces and the body's "fight-or-flight" response. When a plane accelerates or drops suddenly, your internal organs experience a brief moment of reduced weight, causing them to shift slightly; this is perceived by the brain as a "drop." Simultaneously, the excitement or anxiety of flying triggers a rush of adrenaline and norepinephrine, which diverts blood flow away from the digestive system and toward the muscles, creating the sensation commonly known as "butterflies." Another common feeling is "jet belly"—a sensation of being bloated or gassy. This happens because cabin pressure at cruising altitude is lower than at sea level. According to Boyle's Law, as pressure decreases, gas expands. This causes the gas already in your intestines to expand by up to 25%, leading to that pressurized, uncomfortable feeling in the abdomen that many travelers experience on long-haul flights.