In those old days, they cooked in the kitchen with a big kettle that always hung over the fire. Every day they lit the fire and added things to the pot. They ate mostly vegetables and did not get much meat. They would eat the stew for dinner, leaving leftovers in the pot to get cold overnight and then start over the next day. Sometimes stew had food in it that had been there for quite a while.
Hence the rhyme: Peas porridge hot, peas porridge cold, and peas porridge in the pot nine days old.
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