blackout in the blizzard 6x16

Brennan: You know the difference between strength and imperviousness, right?

Booth: Not if you’re going to get all scientific on me.

Brennan: Well, a substance that is impervious to damage doesn’t need to be strong. When you and I met I was an impervious substance. Now I am a strong substance.

Booth: I think I know what you mean.

Brennan: A time could come when you aren’t angry anymore and I’m strong enough to risk losing the last of my imperviousness, maybe then we could try to be together.

- Bones 6x16 (The Blackout in the Blizzard)