The Trouble With Tribbles - S2-E15
Capt. Kirk: I want to know what killed these tribbles.
Dr. McCoy: I haven't figured out what keeps them alive yet.
The Trouble With Tribbles - S2-E15
Dr. McCoy: It is a human characteristic to love little animals, especially if they're attractive in some way.
Spock: Doctor, I am well aware of human characteristics. I am frequently inundated by them, but I've trained myself to put up with practically anything.
Dr. McCoy: Spock, I do not know too much about these little Tribbles yet, but there is one thing that I have discovered.
Spock: What is that, Doctor?
Dr. McCoy: I like them... Better than I like you.
The Trouble With Tribbles - S2-E15
Trader: Four credits.
Cyrano Jones: Is that an offer or a joke?
Trader: That's my offer.
Cyrano Jones: That's a joke.
Answer: Kirk was getting his physical and Dr. McCoy probably turned off communications, because if he hadn't, Kirk would have left and headed straight for the bridge, leaving McCoy irritated.