If you see your cat hissing at another cat outside your window, do not pick up or touch your cat. Your cat may attack you or another animal in the house because of redirected aggression. Redirected aggression happens when an animal is really upset, and doesn't know what to do with that feeling of anxiety, so they lash out at innocent bystanders. So what should you do if your cat is really upset about seeing another animal outside the window? Do not go near him or let your other pets near him. It's best to ignore him until he settles down. If this is happening frequently, you can take some steps to try to reduce the trigger of your cat seeing another cat in your yard. There are motion detectors and other deterrents that you can buy to keep animals out of your yard. You could also shut the curtains so he can't see out.