Two biology grad students walk into a bar.
"Ok, now remember: if an if/then proposition is true, then its contrapositive is also true," says the first.
"Okay, an example: 'If a cell is E. coli, then it must be Gram negative.' The contrapositive would be, 'If a cell is not Gram negative, then it cannot be E. coli.'"
"Ohh, of course," says the second. "Quite obvious. Entirely bacteriological."