Easy way to explain git to new programmers

[fovea1959]

We've been blessed with over 30 new students this year, of which about 10 want to program.

I think it's safe to state we'll be making heavy use of git branch/merge.

Does anyone have a succinct explanation of branch/merge suitable for consumption by brand-shiny-new programmers? I still have 3 sessions of classroom time, and I can spend one of them on this (with labs!)

I'm starting to roll my own based on the Pro-git explanation, but don't want to re-invent the wheel, especially since someone else's is likely to be rounder than mine....