Great teaching example I’ll use this fall, from George Gopen’s Expectations: Teaching Writing from the Reader’s Perspective:
The film has been modified from its original version. It has been formatted to fit your TV.
The film has been modified from its original version: It has been formatted to fit your TV.
(Gopen capitalizes the independent clause after a colon. I don’t, so I would write this without the second capital.)
In the first version, a number of modifications could have been made: change in length, removal of bad words, elimination of explicit sex scenes.
In the 2nd version, just one change has been made, and we know what it is. The film has been formatted to fit our TV. The end.
Gopen says the colon in such sentences functions as an equals sign, and I like that way of thinking about it, though colon-as-equals-sign is too abstract to help students decide when to use one, obviously.
But as an analogy, it’s interesting and fun.