'Is it true that you shouted at Professor Umbridge?'
'You called her a liar?'
'Have a biscuit, Potter.'
'Have - what?'
'Have a biscuit, . . . and sit down.'
-- Minerva McGonagall to Harry in her office (OP12)
Professor McGonagall kept a “tartan” plaid tin on her desk filled with Ginger Newt biscuits (OP12).
She offered Harry some as a way of praising him for telling off Dolores Umbridge, something every teacher at Hogwarts wanted to do. On the other hand she was worried that he would get in trouble again with the Ministry of Magic, since they refused to believe that he had seen Voldemort return in the graveyard (OP12). When Harry said he understood from Umbridge's speech at the opening feast that the "Ministry of Magic is trying to interfere at Hogwarts," McGonagall knew he had actually gotten that idea from Hermione (OP12).
Minerva was much less sympathetic when Harry and the Weasley Twins fought Draco Malfoy after a Quidditch Match. She slammed her desk and Ginger Newts fell out of the tartan tin and onto the floor (OP19). She might have been more sympathetic that time if Dolores hadn't been standing there watching.