A study was conducted with forty-seven preservice childhood (elementary) education teachers (42 F, 5 M) enrolled in two mathematics methods classes taught by the same instructor to determine the effect of these college students making curriculum materials appropriate for upper elementary students on the preservice teachers' knowledge of numeration, algebra, geometry, and measurement. A "Mathematical Mystery Object Box" is a box containing a set of objects with corresponding clue cards. The student reads the mathematical clues addressing numeration, algebra, geometry, and measurement and attempts to locate the object that satisfies them, checking work with the picture of the correct object on the reverse side of each clue card. The study used an identical pretest/post test design with a 20-item instrument tied to the New York State Standards for Mathematics for grades four through six. Preservice teacher scores on the pretest administered the first day of class were low (mean = 54%, standard deviation = 18%). However, without any class instruction or review, but through creating teaching materials for elementary students, the post test scores of preservice teachers increased significantly on the post test given nine weeks later (mean = 71%, s. d. = 14%). This indicates the amount of learning that occurs when preparing materials to teach others. Example mystery object box materials created by the preservice teachers are provided. (Contains 4 tables.)