[T.G]

Dr. Egon Spengler: There's something very important I forgot to tell you.
Dr. Peter Venkman: What?
Dr. Egon Spengler: Don't cross the streams.
Dr. Peter Venkman: Why?
Dr. Egon Spengler: It would be bad.
Dr. Peter Venkman: I'm a little fuzzy on the whole "good/bad" thing, here. What do you mean, "bad"?
Dr. Egon Spengler: Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light.
Dr. Ray Stantz: [low whistle] Total protonic reversal!
Dr. Peter Venkman: That's bad. OK, all right, important safety tip. Thanks, Egon.