The dean of Engineering
had once walked into a class, and said "Good Morning."
The whole class chorused "Good Morning".
"Hi, you are freshmen aren't you?" he asked. One student bolder than the others asked him how he knew.
"Well," he said. "When I say 'Good Morning' to a class, if they are freshmen they say 'Good Morning' too.
If they are sophomores, they quietly fold their papers away, and look at me.
A class of Juniors will look at me over the top of their papers, and then get back to them.
A class of Seniors will ignore my greeting, and keep reading the papers.
When I say 'Good Morning' to a class of graduate students, they write it down.