I found a REALLY fun book, it's called The Superior Persons Book of Words by Peter Bowler and in one of his definitions he posed a challenge and I had fun with it so here it is

Note, the pic really is of a butter firkin, firkins also refer to the casks used to hold beer, ale, or wine but please note that the difference in 9 gallons or the 8 7/8 is if it's used for beer or ale (9 gallons) or wine (8 7/8 gallons) AND "Butter and soap used to be sold by the firkin, too. In these cases it was rather a measure of weight, 56 lb (25.4 kg) and 64 lb (29.0 kg) respectively." ripped from Wikipedia

A great task it is, maybe even monumental
But knowing me it will end up being detrimental
A simple task it is not

The task is to rhyme firkin
With gherkin
And merkin

The problem lies in that they have nothing in common
Matter of fact only one of the words isn’t uncommon
A simple task it won’t be

Firkin has two different meanings
One of them is a really wacky meaning
Okay so they’re both wacky meanings

There’s even some debate on the gallons
Is a quarter barrel 9 gallons or 8 7/8 gallons
A simple task this definitely won’t be

The other one is kinda fun
A small wooden keg, but it holds no rum
And it might even sound kinda dumb

A firkin holds butter
And no I didn’t stutter
Now keep your mind out of the gutter

Now most of us know what a gherkin is
A cute little cucumber it ‘tis
The ones who pickle sure are the whiz

Merkin, merkin, merkin
It definitely ain’t no gherkin
And it sure ain’t no firkin

So what is a merkin you might say
To keep this PG there really is no way
You’ll just have to look it up some day