I just adopted a 12 week old kitten and i have a 3 year old cat. At first I kept them separate but I only live in a small flat with 2 bedrooms so I have to move the kitten into the kitchen over night. I put the kitten in his cat box and let them see each other. They seems fine sniffing each other no hissing and i have them treats. I then put kitten back in his room. Later on I tried it again with no hissing and opened the door. My older cat just watched him and followed him. Seemed to be the longer he was out he got a bit upset and wanted to go outside so i put kitten back in his room again. This morning I let kitten out tje room with my older cat there and they were touching noses but after a little bit my older cat began to hiss and was hitting him with his paw and then he wanted to go back outside. I have asked friends ans they say just to let the cats in together and leave them to it but I'm scared my older cat will not want to come back in the house if he thinks his territory has been taken. What should i do? Kitten is always crying when he's on his own.