How do you write a Limerick?

Question: How do you write a Limerick!?

I get the a-a-b-b-a rhyming scheme, but I don't understand the rhythm part!.

I once took a whore to my Bed,
I couldn't see Nothing but her Head
But when I looked down
I really must frown
For she had a big Little Fred!.Www@QuestionHome@Com

Best Answer - Chosen by Asker:

uh!. !. !. that's really lame - how old are you!? Ten!?
It's not only the rhyme scheme; you have to have the right number of syllables in each line
8
8
5
5
8
There once was a teacher named Sue
Whose sneaker got stuck in the loo
She pulled and she tugged
She jerked and she lugged
And all she could say was, "Oh, poo!"

Ok - that's lame, too, but the rhythm is okayWww@QuestionHome@Com

there once was a fellow named Yates
who could do a fandango on skates
till he fell on his cutlass
which rendered him nutless
and now he is useless on datesWww@QuestionHome@Com