Quantum-locking

Quantum-locking was a defence mechanism used by the Weeping Angels. When observed by any living creature (even other Weeping Angels) the Weeping Angels turned to stone, making them resistant to harm. When they weren't being observed, the Weeping Angels could move. Weeping Angels could also turn to stone at will, or when they believed they were being observed. However if the Weeping Angels stared at each other they would turn to stone permanently, leaving them trapped until they died from starvation. (TV: Blink, Flesh and Stone)