Why doesn't a whole town get sick at once?
A cold going around school in winter: some classmates already had their flu shot (the green 'walls'), so the bug reaches a few kids then fizzles before it gets to the whole class.
Talk about it
- Guess first: how many people in our town would need to be immune to stop a germ — half? most? everyone?
- Why does it still help you if your neighbors are immune, even when you're not?
- Where else does one blocked path stop a whole chain — dominoes, traffic, a game of tag?