Some of these alarms are motions detectors that will sound off when a person or large animal comes into the area. These can protect the yard as well as the pool. The ones that attach to the door or gate have a magnetic attachment that is separated to sound the alarm. One piece is on the door or gate and the other to the wall or fence. They remain together until the gate or door is opened by someone other than a knowing adult. When the door or gate opens, the magnetic pieces separate and set off the alarm.