There once was a man from Nantucket
Whose cock was so long he could suck it.
He said with a grin,
As he wiped off his chin:
"If my ear was a cunt, I'd fuck it!"

Yet another version I heard from schoolmates as a kid:

There once was a man from Nantucket
who found a large frog in a bucket.
He brought it inside
and cooked it up fried
but it tasted so bad, he upchucked it.

There once was a man from Nantucket
Who kept all his cash in a bucket;
But his daughter named Nan
Ran away with a man,
And as for the bucket, Nantucket.

-anonymous

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The well known Old Master of Limerick is Edward Lear (1812-1888), and he required the first and fifth lines of the poem to end with the same word. Most modern limericks ignore this rule, but the above one is a great example of how well it can work.