It happens pretty frequently. A student who you've had in class once or twice emails or stops by your office to ask for a recommendation letter for graduate school. "I've done research with Dr. So-and-so and have done an internship with Dr. Other but I need three recommendation letters for grad school. Can you help me out?" Finding that third letter is often really hard for students. Like most instructors you'll likely feel torn by the desire to help but the knowledge that that a recommendation letter that is simply positive without offering details or information beyond that found in the transcript is not helpful to graduate applications. So what do you do? Should you agree to write the third (likely weak) recommendation letter? As a student, should you request it?