The Night Sky Is a Time Machine
The hook the board draws: every point of light you have ever seen is old news, mailed from somewhere and only now arriving. There is no video for this one, so this lesson carries the whole depth, story first, then numbers.
Before you start. You need nothing you do not already have: dividing a distance by a speed gives you a time, the way it does when you work out how long a drive will take. That one division carries the whole first half.