The formula for writing a Limerick is as follows:
A (long, rhymes with top line)
B (short, rhymes with line above)
A (long, rhymes with top and 2nd line) - this is the 'punch line'!
"There was an old man from Kilmarnock,
Who hated his neighbour's alarm clock.
He kicked down the door,
Then fell to the floor,
When the neighbour, like, stuck out his foot, Jock!"
Not the most brilliant example, but you get my drift?