A: I have to go back upstairs.
B: Why? We're already late.
A: I have to check the stove.
B: What's the matter?
A: Maybe I left the burner on.
B: No, you didn't. I checked the stove before we left.
A: Are you sure?
B: Of course I'm sure.
A: Well, I have to go back upstairs anyway.
B: It's getting later every minute.
A: I think I left the water running.
B: No, you didn't. Let's go! The only thing running is the clock!