Probably their best known book, and a charming one it is, was their collaboration with the illustrator Maurice Sendak: I Saw Esau. Subtitled "The Schoolchild's Pocket Book," it's a compendium of all the rhymes you chanted as a kid skipping rope or dissing a playmate. Plus a whole bunch you had no idea existed.
For example, I never ever heard the counting rhyme:
As Eenty Feenty Halligolun
The cat went out to get some fun.
He got some fun and tore his skin
As Eenty Feenty Halligolin.
The Opies point out that this rhyme bears the faintest traces of pre-Roman, Anglo-Cymric (Welsh that is) numerals. Eenty, feenty == one, two?
Here's a funny one that marches along just for fun.
Oh the gray cat piddled in the white cat's eye,
The white cat said "Cor blimey!"
"I'm sorry, Sir, I piddled in your eye,
I didn't know you was behind me."
And did you ever chant this one?
The cat's been sick.
Under the chair.
Fetch the basin.
It is too late,
The carpet's in
An awful state.
It's all in vain,
For she has licked it
Per this last: Children are nothing if not observant!