elbow grease

Christopher and I were cleaning our new "used" fridge a few weeks back. He loves to help me do stuff (if only he had the ability to mow the yard!)... Anyways, he's scrubbing away at the crud, but it's not coming off (for him). He said, "Daddy, this stuff is not coming off." I told him not worry about it - that I would have to put some "elbow grease" on it. Immediately, he held out his little hand and said, "Well, here, give me some."