I'm going through a corny joke phase and as such, I'm loving the A Prairie Home Companion's Annual Joke Show transcripts (from the mostly science pita Simcoe).

Thinking about jokes reminded me of a conversation some years back that I had with my some of my friends from high school.  We discovered that all our Moms could not tell jokes. We hypothesized that it was the result some sort of biological change. This idea resulted in the creation of this imaginary scene in a doctor's office somewhere:

woman: 
"Doctor, I'm constantly feeling nauseous"

doctor: 
"Knock knock."

woman: 
"What are you doing? I'm right here. Why are you knocking?"

doctor: 
"Please pee in this cup."

Most kids go through a joke phase. As for myself, when I was little I just couldn't get enough of the corny jokes of The Muppet Show or Wayne and Shuster.

I've recently read somewhere that jokes are a good indication of verbal comprehension because if a child can't pick up the implicit double meaning of a punch line, the child can't enjoy the joke. So when a kid reaches his or her joke phase, that child has reached a certain level of understanding of words and of meaning. 

On that note, I believe "dirty jokes" work the same way: if you don't understand the context, you can't understand the joke. 

It's funny - as I have been writing this I have been considering including my personal favourite dirty joke -  but because it involves a play on words, there's no point because it has to be told and not read to be funny. 

As far as I'm concerned, this is another great thing about jokes.