Quantum-locking

Quantum-locking was a defence mechanism used by the Weeping Angels. When observed by any living creature (even other Weeping Angels), Weeping Angels turn to stone, making them resistant to harm. When they aren't being observed, Weeping Angels can move freely. Weeping Angels can also turn to stone at will, or when they believe they are being observed. However if Weeping Angels stare at each other they will turn to stone permanently, leaving them trapped until they die from starvation. (TV: Blink).