In my senior year high school physics class I thought I was going to fail the final exam because I couldn’t remember all the formulas. We were allowed to bring a calculator to the classroom for the test. I had the kind that had a soft black plastic cover that folded over the front. I realized that I could write the formulas in the inside of the cover using a black pen, so they wouldn’t be visible except at the right angle when the overhead fluorescent lights in the classroom would cause them to reflect in a slightly reddish tint. So I used a principle of physics in order to pass my physics exam.