We've been slowly introducing our new 10 week old kitten Jessie to our 14 week old kitten Luna. To start with there was a fair amount of hissing but no physical contact of any sort. A couple of days later and they seem to be playing, but I worry that it might actually be fighting. Luna still growls and hisses occasionally but she's been laying on her back and pawing at Jessie. Jessie just sits and looks at her as if she's crazy most of the time but does also chase her around and paw at Luna too. The thing that worries me is the hissing and growling. Luna has always been extremely vocal so I think it might just be her. Jessie only ever hisses when she's had enough of Luna. Luna doesn't seem to know when to stop and leave her alone. But then she's never known when to stop even when playing with us!