How Speeds Normally Add
▶ DRILLVika walks toward the front of a moving train. To George on the platform, she's going the train's speed plus her walking speed — and if she turns around, the train's speed minus it. Speeds just add up. It's so obvious nobody ever questions it, which is exactly why light — which flatly refuses this rule — comes as such a shock later. Use the Explain it for a kid / a grown-up switch to set the depth.
The big idea — to find how fast George sees Vika go, you just add the train's speed and her walking speed (or subtract, if she walks the other way). Everyone does this without thinking. It works perfectly for trains, balls, planes — everything you'll ever throw or ride. But there's exactly one thing in the universe that breaks it: light. Hold onto this rule — the surprise comes when we try it on a beam of light.