The Tooth Fairy will take the tooth away while the child is sleeping and leave some money in its place . The tradition is still very much alive and well in Ireland and Great Britain , where it is common for young children to believe in the Tooth Fairy . When a child 's sixth tooth falls out , it is customary for the tooth fairy to slip a gift or money under the child 's pillow , but to leave the tooth as a reward for the child growing strong .