Years ago, we had had a house cat for a couple of years when someone talked us into taking a kitten. We didn't want a fight between the resident cat and the "invader," so we put the kitten in a spare bedroom. We figured we'd give the kitten a few days to get comfortable with "his" space, and introduce the two of them with limited, supervised visits. Snicker (the "cat in possession") knew there was another animal behind that door, of course, and he kept sniffing at the crack beneath the door. One time, I was in the room with the kitten, and I heard Critter say, "Snix, you want to see what's in there?" and he opened the door a couple of inches. The kitten was near the door at the time; I was a good bit further away. I don't know what Snicker thought he saw, but he hissed explosively and took off down the hall as fast as a cat can run. My husband found him behind the couch, every hair on end and growling so wildly that Critter was afraid to touch him (did I every tell you how Snicker got his name?!). Introductions between the two cats went reasonably smoothly, other than the fact that every time the door was opened, Snicker took off for fear the "whatever" was going to come out rather than the kitten. For months afterward, if we left the door open, we'd see Snicker peeking around the frame, wide-eyed and nervous. (BTW, this is the same cat that demolished the Christmas tree).