In 2267 our fluffy tribble pals invade the Enterprise NCC 1701. Trouble with Tribbles is a great episode because this harmless little tribble critter is the center of much devotion on the U.S.S. Enterprise 1701 and the Deep Space station K7. Spock being vulcan, naturally, wouldn't admit to being affected by the little animals trilling.
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"Most curious creature captain. Its trilling seems to have a tranquillizing effect on the human nervous system, fortunately of course, I am immune to its effects." Spock's crew mates looked at him in amused, sheer disbelief. What makes this really funny is that Spock remarked on the creatures trilling while enjoying the tribbles company, which he held patting in his hands the whole time with much fascination and affection.
Live long and Prosper Trekkers! You find more about this phenomena at Tribble Trouble.