Lines one, two, and five MUST each have exactly three metric feet

Lines three and four MUST each have exactly two metric feet

The metric feet MUST be anapests ( da da DUM ) although the leading foot of each line may be an iamb ( da DUM) and the last foot of each line may have a trailing unaccented syllable ( da da DUM da).  The classic Limerick is consistent in the use of iambs and trailing unaccented syllables, but this is not mandatory in recreational Limericks.

Lines one, two, and five MUST rhyme

Lines three and four MUST rhyme

A good Limerick will have a clever, unanticipated punch line as line five

A good Limerick will not be insipid or pointless

A good Limerick often has puns, word play, eccentric spelling, or some other witty feature

Any nonsense poem that lacks five lines, thirteen metric feet, or the aabba rhyme pattern is simply not a Limerick.  It might be a sing-song or a la-de-da, but it's not a Limerick.

There ONCE was an OLD man from WHEEL-ing

Who HAD a pe-CUL-i-ar FEEL-ing

Said the SIGN on the DOOR

Please don't SPIT on the FLOOR —

He JUMPED up and SPAT on the CEIL-ing.