On various occasions, unusual circumstances have rendered some or all of Susan's powers useless: In "Hogfather", she becomes completely human and finds herself bereft of her inherited gifts upon entering the Tooth Fairy's castle. This is because the laws of reality within the castle are based on the beliefs of children, which do not include a concept of death; hence the part of Susan which is derived from Death does not exist there. In "Thief of Time", Susan finds herself unable to walk through walls while time is being held in stasis. She speculates that the energy of the glass clock which is holding time static is in some way interfering with her power.