Not that one. I'm talking about the one in written language. The colon is a tricky punctuation mark. Somehow, someone got it into everyone's head that the colon is best used before a list. Well, it can be used before a list, but only under certain conditions.

The confusion, I think, comes from the colons regular use in headings. We often see a colon after a heading, indicating that the information that follows explains and illustrates the heading.

Colons:

You expect what follows to be a list or other information about colons. And that's as it should be. However, outside of their use in headings--meaning when you are writing complete sentences--colons must be used after a complete sentence. For example,

I went to the grocery store for: bread, cheese, and milk.

This sentence is incorrect. A colon is absolutely not necessary, and wrong, here. Why? Because the information before the colon is not a complete sentence. Look:

I went to the grocery store for.

That doesn't make sense. "For" sets up the need for the items that follow and so that word is already doing the work that we imagine the colon would do: connecting the list to the rest of the sentence.

Using a colon after "were", "was", "including" and a number of other words has the same effect. Essentially, it creates a sentence fragment before the colon and an awkward list after it.

Here are some incorrect uses of the colon:

The requirements were: memory, battery life, and weight.

We purchased new computers, including: hard drives, screens, and keyboards.

Each computer has: a touch screen, a camera, and a 16gb hard drive.

None of these examples is correct. Let me repeat that: not one of the above examples is the correct way to use a colon because colons are not used to set up lists. Colons are used when what comes after the colon (which need not be a complete sentence) explains the main idea in the complete sentence before the colon. So, colons are only used after a complete sentence when the information after the colon explains the main idea of the complete sentence.

Here are some correct ways to use a colon:

I went to the grocery store: I bought bread, cheese, and milk.

I went to the grocery store because I needed a few things: bread, cheese, and milk.

When I went to the grocery store, I ran into someone: your mom.

Colons can be tricky, especially when someone has been spreading that nasty rumor about the colon’s relationship with lists. It's not the list that gives the colon meaning; it's the relationship between the information after the colon with the sentence before it--the fact that what comes after the colon explains the sentence before.