I suppose this requires some explanation:
My brother and his family, including his baby, were visiting my house a few days ago. While my dad was walking the baby around, I stepped out of my room to use the bathroom. My father started directing my nephew towards me, saying "Oh, there's Uncle Voltaire! Let's go bang on his kneecaps!" (the kid really likes smacking stuff). Before the two could reach me, however, I entered the bathroom and closed the door behind me. From the other side, I could hear my dad say, "Oh, well; his kneecaps are safe for now," to which I responded: "My kneecaps are always safe!"
And it sounded like a fun idea for a drawing.

I might make this a little series, where I take random phrases I or other people say and make pictures out of them, but without any sort of context; I've already got another one in mind.