Limericks are short poems not meant to be taken seriously.
They consist of five lines with rhyme scheme aabba.
The pattern of syllable stresses is not set in stone,
but it is generally anapestic (unstressed, unstressed, stressed).
The first, second, and fifth lines contain nine syllables,
while the third and fourth have six.
This is a dry and technical explanation for a poem which is,
in reality, simple and fun to write. Limericks frequently
make slight departures from the ‘proper’ form,
and this is perfectly acceptable.
The limerick below gives you an idea of what limericks are all about.

There was once a young fellow whose brain
Was the size of a cereal grain
But his shortage of wit
Was a great benefit
For he failed to register pain.