Possessing a natural and unique defence mechanism, the Weeping Angels are quantum locked. This means that they can only move when no other living creature, including their own kind, is looking at them. As soon as they are observed, they instantly turn to stone and cannot be killed. They are also able to transport other beings into the past, allowing them to feed on the potential time energy of what would have been the rest of their victim's lives. The Doctor must always remain on his guard against these beautiful but merciless killers.