Problem 1 was adapted from a problem in One Equals Zero and Other Mathematical Surprises, by Nitsa Movshovitz-Hadar and John Webb (Berkeley, Calif.: Key Curriculum Press, 1998). Problem 2 is adapted from a problem from the Mathcounts 1998–99 Handbook. Problems 3–14, 27, and 28 come from the mathschallenge.net Web site. Problem 15 is from the Math Forum's Problem of the Week Archives, available at mathforum.org/pow. Problems 16–25 came from—or were adapted from—problems at the Mathematics Problems and Warm-ups Web site, at www.geom.umn.edu/~lori/mathed /problems. Sources of the problems, as cited on the Web site, are as follows: problems 16, 18–21, and 25 are from Mathematical Games, by Marie Berrondo (Englewood Cliffs, N.J.: Prentice Hall, 1983); problems 17, 22, and 24 are from Fun with Brain Puzzlers, by L. H. Langley-Cook (Greenwich, Conn.: Fawcett, 1965); and problem 23 is from the Minnesota Math League. Problem 29 comes from the “Nets of Cubes” problem set from Problems with a Point, a National Science Foundation–supported project at Education Development Center, available at www2.edc.org/mathproblems.