What Does “Moving” Even Mean?
▶ DRILLVika drops a ball inside her smooth-gliding spaceship and it falls straight downto her feet — exactly as it would if she were parked. George on the ground sees that same ball trace a curved arc. Both are right, and no experiment Vika can do inside her sealed ship will ever tell her she's moving. So “moving” isn't a thing on its own — there's no special, stationary place to measure the universe from. The whole theory starts here. Use the Explain it for a kid / a grown-up switch to set the depth.
The big idea — you can never feel smooth, steady motion. On a calm-flying plane you can pour a drink, drop a ball, play catch — all exactly as if you were standing still. So "moving" isn't a thing by itself — something is only moving compared to something else. Galileo noticed this on a ship 400 years ago. Einstein's leap was to insist it must hold for everything — light included — and that one stubborn move forces all the strangeness in the next drills.