There is, of course, the way they cock their head to one side, as if to say, "Really?!" And their warm, almost human, eyes. Their jauntiness and energy.
With Copper, there are his antics: racing around the house in circles, begging on his hind legs like a circus dog, thumping his tail on the floor then barking because he doesn't realize that he just made that noise and not an intruder.
With Motet, who's visiting us for a while, it's her big ears, her tiny little body (she weighs less than 10 pounds soaking wet), her wiry coat (looking as if she just threw on a robe out of the bath) and her sweet little face.
Whatever it is, it's cute.