What is the rule for limericks?

Each verse consists of 5 lines. The first 2 lines have to have enough syllables to be counted as 4 beats each (3 beats in which your saying something, then a 1 beat pause). The 3rd and 4th lines count as 2 beats (with little pause between), and finally there's one last line that's counted as 4 (in the same fashion as the first 2). The first, second, and fifth lines will all rhyme with each other (not with the third and fourth however), and the third and fourth lines will also rhyme (not with the 1st, 2nd, and 5th obviously). There's a famous limerick: "There once was a man from Nantucket" that shows the structure well.