Why do kittens sometimes get poofy and hiss when they meet a strange cat? I could understand those scaring off members of other species but fellow cats? https://www.youtube.com/watch?v=IJXMFr5lf80; https://www.youtube.com/watch?v=mnVLjKRCO4I

I've been around kittens for years and yes they do have the instinct to hiss at any cat they feel is invading they're territory . This is totally normal kittens hiss to protect they're self from a adult cat . For example you have Mother cat with babies and Male cat comes around she's going to protect her young especially if the Male cat isn't the father of the baby kittens. She is going to fight him off and the baby kittens will hiss at the Male cat to chase him away from them yes it's happened in the past i been around enough animals i do know how they act.