From the text: Third-grade detectives made a fun game of investigating common subtracting errors in teacher-made examples, as well as in peers\rq work and in their own. The action research process began with some background readings about common error patterns in subtraction, and the teacher began formulating ideas about how error patterns could be used when working with third graders. Initially, she administered a twenty-problem pretest comprising two-digit subtraction. She began her instruction of subtraction by telling her students that they were going to be subtraction detectives. They were each given base-ten blocks and a magnifying glass. The day\rq s lead investigator was given a detective\rq s hat to wear.