The weight you feel right now is not gravity but the floor pushing up against you. This episode starts there and builds a working explanation of g-force as a measure of acceleration, felt only when one surface presses against another. It walks through why upward acceleration feels identical to a heavier planet, why an unconditioned person sitting upright loses consciousness around four to five Gs, and how gray-out, tunnel vision, blackout and G-LOC follow as blood drains from the eyes and brain. G suits and straining breathing let trained pilots sustain nine Gs, while negative Gs of two to three cause red-out because jugular veins lack one way valves.

The story then moves to the extremes. In 1954 John Stapp rode a rocket sled to Mach 0.9 and stopped in just over a second, enduring a peak of 46.2 Gs and living to 89. Racing driver Kenny Bräck survived 214 Gs in a 2003 IndyCar crash because the spike lasted a fraction of a second. Beyond humans, the mantis shrimp strikes at 10,400 Gs, jellyfish stingers fire at over 5 million Gs, and bacteria kept growing at 403,627 Gs in a Japanese ultracentrifuge.

• Eyeballs in versus eyeballs out: why lying down changes tolerance
• Sustained acceleration versus jerk, and how crumple zones stretch out a crash
• A hearty back slap at 4.1 Gs and a sneeze at 2.9 Gs, compared with 3 Gs on the space shuttle
• Cavitation bubbles that let a mantis shrimp stun prey even when it misses
• Nematodes surviving 400,000 Gs for an hour and what that suggests for panspermia