My young son was doing sack races on our hardwood floors this morning, using our soft, fuzzy flannel pillow cases as the sack.
You can probably guess what happened next.
My young son crashed into the hardwood floor, yelling that "Ow!" that parents for blocks can hear. My older son ran to him and scooped him up in his arms.
"Oh no, you fell? Your coefficient of friction was too low... (I didn't hear the next part because I was processing the sentence)... You know your friction would be larger if your mass was larger... (young son is quite skinny)."
Young son snuggles in deeply into old son's arm, feeling comforted. Between sniffles he mutters, "I didn't (sniff) have enough friction (sniff)."
I leave the two sons to talk for a minute and as I walk away I hear, "You know, normal force equals mg. You were standing up; you weren't sticking to the side of the wall." They laugh, sharing their secret geeky jokes as they ease the pain.