Q: "Crypt Rats" give their owner the ability to "tap X: Crypt Rats deals X damage to each creature and player." Can this ability be used as often as desired? Can it be used defensively during another players turn? Can it be used even if the "Crypt Rats" card is tapped? Presumably, the Crypt Rats themselves are immune to the damage caused by this ability?

A: The ability can be used at any time, but the Rats are not immune to the effect. This limitation usually means the rats' ability is only used once.