sentences:  However, the metals they use must be untouched by darklings or slithers for them to be effective.To maximize the effectiveness of the Midnighters' weapons, they are often given names with 13 letters, or names with a number of letters equal to a multiple of 13, and are sometimes inscribed with certain patterns representing mathematical concepts.The names given to objects can only be used once, but the metal can be used several times, depending on how strong a darkling it was used against.