Abstract: A liquid sensor device, system and method depends on the structural deformation of a trigger material in contact with a liquid, such as water, to convert potential energy of a resilient material to kinetic energy that triggers a mechanical response in an activated state. A transfer arm is biased toward the activated state and may be retained in a deactivated state, against urging of a spring, by a trigger material that may be structurally rigid while substantially dry. Upon contact with a liquid, such as water, the trigger material may relinquish structural rigidity and be substantially deformed. Upon substantial deformation of the trigger material, the transfer arm may be urged into the activated state by activating a locking mechanism or electronically, such as by activating a switch such as a micro switch. In the activated state, the triggered mechanical response may activate an alarm, and may trigger an electrical response to close a valve, such as a natural gas valve on a gas-heated hot water tank.