An English teacher spent a lot of time marking grammatical errors 
on her students' papers and was beginning to doubt whether she 
was getting through to them.  One day as the stress got to her, 
she leaned over her desk and rubbed her temples.

"What's the matter, Mrs. D?" one passing student asked.

"Tense," she mumbled.

The student hesitated a moment, then said, "What could be the matter?
What has been the matter?  What was the matter?"