Pease-porridge hot,
Pease-porridge cold,
Pease-porridge in the pot,
Nine days old;
Some like it hot,
Some like it cold,
Some like it in the pot,
Nine days old.

pease porridge = pea pottage, or soup

at some point pease was both the singular and plural form and it was a short step from there to the singular pea.

"envision whirled pease!" -anon