The Trier guide for students tells of a professor who at first said du to her staff. Over the years she found that younger staff found it increasingly hard to say du to a respected and older superior. She also found that it is much easier to criticize staff, to warn them or even to dismiss them, when one is on Sie terms with them. According to the guide, most professors therefore switch at some point in their careers to an obligatory Sie with new staff, if they have not used it from the start. The guide also says who makes the first move. A research assistant would hardly offer the du to the head of department, but the other way round would be more conceivable.