Recently while I was at the front doors of the School greeting students and parents at the beginning of school, I noticed a father with his second and fourth grade children approaching. The father recently changed jobs and was about to begin his first day of his new job. So, as the three of them entered, I said to the father with a wink of the eye, "Hey, David, break a leg." He smiled and said, "Thanks." Immediately the second grader looked up at Dad and said, "Why would Dane tell you to break your leg?" I had to trust Dad would explain what an idiomatic expression is. In case you forgot, check out this great site: Using English.com.