Why a Siren Changes Pitch
▶ DRILLVika's siren races past. Its sound waves bunch up in front of her and stretch out behind — so George ahead hears a high EEEE↑ and Liza behind hears a low ewww↓, even though the siren itself never changed. That's the Doppler effect, and light plays the exact same trick with colourinstead of pitch — the groundwork for redshift later. Use the Explain it for a kid / a grown-upswitch to set the depth.
The big idea — when something making a sound moves, its waves get squished in front and stretched behind. So a siren sounds high coming toward you and drops low as it passes — even though the siren never changed. Light does exactly the same trick, but with colour instead of pitch: bluer coming closer, redder moving away. That's how we know far-off galaxies are racing away — a drill for later. (And if the siren goes faster than sound, the waves pile into a wall — a sonic boom.)