Instructions
Open windows only an inch or two so your cat has no chance of getting through.
Install strong screens in your windows to help keep your cat in if you need to open your window any wider. Screens come with the added benefit of keeping insects out. Consider getting pet screen, which, while more expensive, is also stronger and will better protect your precious pet.
Check your screens regularly for damage and to make sure they're securely fastened in the window. A poorly secured window screen can fall out, and your cat along with it.
Place tinfoil along the window sill if you don't want your cat sitting near the window. The sound and feel will reduce the likelihood that the cat will jump up there in the first place, though there's no guarantee that a stubborn cat won't ignore the annoyance to get to its favorite perch.
How to Make Windows Safe for Cats
An old folk saying says that cats always land on their feet. Though often true, a cat's ability to right itself doesn't help it prevent injury, and serious falls can lead to injury, broken limbs and even death. Open windows present a serious danger to cats, which are more than happy to jump out of them in chase of birds, bugs, fear or their own mysterious whims. Make sure your windows aren't posing a hidden threat to your cat's safety.