Prof. Lachner once taught a class from 2:30pm to 5:30pm. Every time the
class met, all the students would have a lot of food on their desks when
the class started. During the 5 minutes break, all of them would line a
queue at the nearby vending machine. He couldn't understand why these
students were hungry all the time, anyway, his class was just after the
lunch time and long before dinner time. Prof. Somebody was not happy about
this because when they ate, they make a lot of noise. So he announced one
day "No food in the class". Next class he found the classroom extremely
quiet. Guess what, everybody was dozing because nothing was keeping them
awake.