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.
This nursery rhyme reportedly derives from medieval stew-pots hanging over the fire with whatever food (mostly vegetable) was available added, so it was possible that some item could be nine days old. This is widely circulated in the "Life in the 1500s" e-mail that is found archived on numerous web sites.
Even snopes.com admits, after debunking most of the things in the e-mail, that "Even some cooking practices of today call for tossing whatever's on hand into the stewpot, with new ingredients added each day to whatever is left over. French bouillabaisse, for instance, is sometimes made this way, as are any number of 'peasants' stews.'" However, peas were usually stored in dried form, and so had to be cooked quite a while anyway, like dried beans, before they could be eaten.
"Pease" used to be a singular term for any number of the vegetable; "pea" is a back-formation from it, and the spelling was changed to "peas" to look like a plural noun.
Sources:
http://www.snopes.com/spoons/fracture/1500.htm
http://www.soupsong.com/fpeas.html
http://www.foodmuseum.com/pea.htm