Rule no. 3
Minerva McGonagall was about to pay a visit to Hogwarts' kitchens to get herself a strong cup of coffee (she was to have the Weasley twins in her class the next hour) when the Divination professor blocked her way on the stairs.
'Minerva! I am receiving a sign, it tells me that you are off to the kitchens.' She said, her giant eyes gazing absently but still sparkling.
McGonagall sighed deeply and replied 'Sybill, hello, that is exactly where I am going, indeed. Did a little bird inform you? Or perhaps the fact that this stairs only leads to the kitchens helped you with that prediction?'
Trelawney simply shook her head and replied 'oh, Minerva, such a smart woman but so short sighted… a pity, really.'
McGonagall inhaled deeply and tried to keep her calm. She couldn't bare to be short tempered when having to deal with the Weasley twins. Her head was full of compromising comments and comebacks but she decided to stay civil.
She tried to sneak past Trelawney. 'Would you mind, Sybill?'
'Of course not, Minerva, go ahead. Remember not to use any milk, though!' The "seer" told her as she moved to the side so McGonagall could continue her way to the kitchens.
As soon as she entered the kitchens a house elf called Gibby ran up to her. 'Mistress McGonagall! What may Gibby do for you?' The elf asked.
McGonagall smiled, house elves and their immortal excitement to be of service always managed to cheer her up. 'A coffee will do, Gibby.' She replied.
Gibby nodded 'would Mistress care for something in it?'
McGonagall wanted to reply that it wasn't necessary but then made up her mind. For the sake of proving Sybill Trelawney wrong she answered 'a little milk in it would be nice'.
The next morning, the Deputy headmistress woke up to an unpleasant surprise. As she made her way to the bathroom in her quarters she had passed a mirror and almost drawn her wand.
What she saw in the mirror was not something she was used to seeing, it definitely looked like it but there was one difference: the skin of the being she usually saw in the mirror, her skin, had turned blue.
She had stood there, paralyzed, for a few minutes, staring at her horrifying reflection, when she decided it might be a good time to pay a visit to the Hospital Wing.
She took a glance at the old wooden clock to the right of the mirror and saw it was still early, if she went to visit Poppy right now she might be lucky enough to not walk into anyone.
She quickly changed into her every day robes and made her way down to the Hospital Wing. She was lucky enough to pass no one but two Slytherin fifth years, who were too busy with each others lips to pay any attention to the blue coloured Transfiguration teacher who hurried past them. Once she got to the Hospital Wing she as much as ran toward her colleague's desk.
Pomfrey had not noticed her entrance and so she scraped her throat. The nurse looked up and tried not to burst out into laughter at the sight in front of her.
McGonagall crossed her arms defensively 'Poppy, would you mind acting like a professional? Who knows, perhaps I got poisoned and I will crash dead on your floor in a matter of seconds. Imagine what that would do to your reputation.'
Pomfrey inhaled deeply and her professional attitude returned. 'I wouldn't worry too much about that, dear. The colour of your skin is not going to get you killed. [***] I'll give you something that will make your skin go back to normal within an hour or two.' McGonagall sighed in relief.
'Let me guess, though, you had milk in you coffee yesterday?'
McGonagall nodded.
Pomfrey let out an exasperated sigh. 'I asked Sybill to warn you! Did she forget to do so? That woman…'
McGonagall stared at her shoes in embarrassment, and this time not because her skin was as blue as a cloudless summer sky.
'She might have warned me…' McGonagall said quietly.
Pomfrey looked even more annoyed than she did before. 'Then why, in Merlin's name, didn't you listen to her, Minerva.'
McGonagall felt herself go into defence mode.
'Excuse me? How am I supposed to trust anything that fraud of a woman says? She's insane and I can't believe Dumbledore hasn't fired her already! Nothing that ridiculous woman says makes sense!'
Pomfrey opened her mouth to calm her friend down and to warn her that someone was standing in the doorway, but she was interrupted.
The usually so very neat and strict Head of House had picked up a bright pink scarf that had been laying on the nurse's desk and tied it around her head. She spread her arms and exclaimed 'USE YOUR INNER EYE TO SEE THE FUTURE. WE ARE ALL DOOMED!' as an impression of the Divination teacher.
…
The Divination teacher who was the person standing in the doorway at the very moment.
When McGonagall walked into the Staffroom an couple of hours later, a new rule had been added to her list:
Tying a scarf around your head and screeching "USE YOUR INNER EYE" is a poor imitation of Sybill Trelawney and not in any way funny.
A/N: I think it's fair to say that I went a little out of character at the end of this one. Just a little… also, please take liberty in correcting me if I make any recurring grammar mistakes. I don't have a beta and English is (surprise surprise) not my native language.
***Unless you are an innocent middle aged African American male wearing a hoodie while walking down the street late at night.