Described herein is a science, mathematics, engineering, and technology (STEM) secondary lesson created by graduate engineering student Safa Herfat, with modifications by her co-authors. The lessons learned from this case study are explored through an explanation of tolerance, a description of the lesson, the results obtained, and participant perspectives. Additionally, the fresh approach of real-world applications is explained. Twenty graduate level engineering students (including Herfat) from the University of Cincinnati (UC) created STEM lessons between 2007 and 2011 over the course of their academic years. Tolerances in Engineering Design was a three-day secondary STEM lesson to introduce the critical engineering concept of tolerance. The main objective of this STEM lesson is to make students understand the necessity of tolerances in engineering design, the required tolerances for functional design mechanisms, and the significance of tolerances while designing medical devices and implants. (Contains 4 figures.)