Colon Strategy 1: Use a Colon to Connect Related Sentences

You can use a colon to connect two related complete sentences if the second sentence emphasizes, summarizes, or explains the first sentence.

Correct use: Mr. Lawrence was attacked by a wolverine: his birthday was not going well.
This example works because the second sentence adds to the ideas of the first. Grammatically, it makes sense either to use the colon or to have two separate sentences but by using the colon you can show a direct relationship between the two sentences. 

Not once did I say Mr. Lawrence was human: he was, in fact, a very successful bird-lawyer.

Incorrect use: Mr. Lawrence was attacked by a wolverine: it was snowing on Mt. Fuji.
This example doesn’t work because both statements–while they are true and complete sentences–do not have any relationship to each other.