Upon arrival at the lab, the first thing you will probably need to so is ensure there are enough seats for the students scheduled to attend. In most cases this will mean announcing to everyone in the room that a laboratory is about to start, and anyone not involved with that laboratory should please leave. For those that ask, it it is fine for them to try to return to empty seats in about 10 to 15 minutes. Occasionally, some students will ignore your request, and you may find these later when you take attendance. If you are sure they have ignored you, and there are students for your laboratory waiting, feel free to be ``very cross'' with them.