The punctuation mark, that is, not the part of human anatomy. A post on that would be all kinds of gross.
Specifically, here’s how I teach the use of the colon: I made up a formula that defines its function.
Did you see that? I demonstrated it in that last sentence. What exactly does the colon do? It points to the rest of the sentence; it says, “And now, here’s the exciting conclusion to the situation set up in the first half of the sentence!”
The formula that I present to students is below.
: = →
Colon equals arrow. Bonus: it looks like an emoticon. That dude has warped nostrils, but at least he seems happy.
Consider this: The answer is this four. How would you punctuate that? By putting a colon after the word “this,” of course. The answer is this: four.
How can you know that’s right? Picture the colon’s equal in its place: The answer is this → four. Makes perfect sense.
Gently Hew Stone 
As a writing tutor, I applaud your explanation. Now you have to explain dashes and tell us the difference between dashes and colons. :-)