[The Dark Knight] I'm an idiot. When Fox is explaining the Sonar Phone to Bruce, and Bruce goes "just like a..."

Lucius cuts him off to say "-like a submarine, Mr. Wayne, like a submarine."

I always thought it was sort of a weird analogy to make, and I didn't understand why Lucius would interrupt him like that. But today it finally dawned on me...

Bruce was about to say "just like a bat."