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: Total protonic reversal! 
Dr. Peter Venkman: That's bad. Okay. All right, important safety tip. Thanks, Egon.