Quantum-locking

Quantum-locking is a feature gained by species through evolution. The only known species to evolve this ability were the Weeping Angels.

If a creature is quantum-locked, then they are turned to stone whenever they are observed. However, if the observer turns away, blinks or stops observing them in any way, the creature can move for the time until the observation resumes. This is used as a defence system. Between observations, Weeping Angels would attempt to attack. The Doctor said that 'You can't kill stone,' this is highlighting that quantum-locking is one of the most perfect manners of self-defence.

If two creatures are quantum-locked and are facing each other, they must be careful to cover their eyes before they are turned to stone. If they don't, they risk looking at each other, and being that they are both being observed by the other, they would be trapped in stone form. (DW: Blink)