Kids have the cutest way of mispronouncing words and phrases. Since my "baby" is 16 now I don't get to hear those sweet mistakes anymore. The last one I remember, many moons ago, was him adding the "oh" to the word doorbell.
Yesterday I was pleasantly surprised to hear him use the expression "rule of thumb." We were discussing what time he should arrive home from school, and at what point he needed to call and say he was on his way. He said, "Let's make a rule of thumbs" and then went on to propose his rule. I asked him what kind of rule he wanted to make, and he repeated "a rule of thumbs." Did I correct him? No way.