It is March 13th and you just left the beach with your friends carrying along great memories of a fabulous Spring break. As your thoughts drift from sun-drenched beaches to mass transfer problems, you notice flashers behind you. You can’t believe it—it is that same sheriff that you encountered on your way down—what now? You stop and reach for your license and registration, but the sheriff goes: “No need for that. I am sorry to have to stop you again, but you were so helpful a week ago. I am hoping you can help me again solve a missing person case.” Eager to help (and, perhaps, avoid a fine for the broken taillight) you accompany the sheriff to the private office of a missing lawyer, one Billy Fick. It seems that he may have been involved in a money laundering operation and found himself in the wrong place at the wrong time. Determining when he was last in his office would help incriminate a suspect, one Stefan Schmidt. While looking around, you notice a half-empty coffee mug on the missing lawyer’s desk.

You measure the distance of the liquid surface from the top of the mug and find it to be 6 cm. A neat mark on the inner wall of the mug shows that the initial level of coffee was 1 cm below the rim and that no coffee had been drunk from it. Billy was the only person with access to his office and nothing has been touched since his disappearance. It is obvious that he left in a hurry after placing his mug on his desk and never returned. The temperature and relative humidity in the air-conditioned office are 20°C and 60%, respectively. When did Billy leave his office?