Setup: We have been reading a kids book on the periodic table which includes facts about each element.
Tonight:  I am telling the boys a bedtime story where they are traveling on a train inside a mountain, and they happen upon 2 dragons they have to defeat.  I ask each of them what weapon they will use to defeat their dragon.
4 Year Old: "I will use my Moose Poop Gun!"  So in the story I have him launch moose poop at the dragon, who accidently ignites the poop, and burns himself up in the process.  Now it is his brother's turn to select his weapon again a dragon.
6 Year Old: "I will use my Mercury Bazooka!"  "I will poison the dragon with the element Mercury!"
Well, I guess he was paying attention to the periodic table book details after all.

6 Year Old: "Mom, the word 'mastered' means you have become really good at something, you have become an expert!"
Me: "You are correct, it does, can you use it in a sentence?"
6 Year Old: "I have mastered building with Legos"
Me: "You certainly have!"

At the park and the 6 year old is climbing a structure made of monkey bars in a vertical loop, he tries to hang on with one hand off of the top, but quickly drops down and lands in the mulch.  He then declares "these bars do not have enough texture to grip them, I am moving over to the monkey bars that have the textured pattern on the handles, they are designed to help me grip onto them"   Yes they are my son.