its a very important safety tip from Dr.Egon Spengler of Ghostbusters
"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 fuzzy on the whole good/bad thing. 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: Right. That's bad. Okay. All right. Important safety tip. Thanks, Egon"
also one from Janine "Ghostbusters, what do you want?"