The Staywell Magnetic Cat Door provides security against intrusion by strays without requiring batteries or house current. The secret- The collar key itself is a magnet that actually unlocks the flap allowing access by your pet but not by other pets or strays. The Staywell Magnetic Cat Door is suitable for cats up to 15 pounds. This cat flap is self-framing from 0 to 2 1-4. Because its self-framing it may be installed in any door including solid or hollow core aluminum or other non-magnetic material. It should not be installed in a door made from steel or other magnetic material.