Mycroft watched his brother with his hands raised; prepared to cover his face in case anything went… not as expected.
-You can leave me now if you're so scared, Mycroft, you are aware I just wanted to borrow your book; I do not need your help with this. Do you not have some food to go eat?
-Even though I stick with my position that you are an insufferable child, Sherlock, you are my brother and I wouldn't like to see you end up in the infirmary. Nor I would like to have to deal with mummy afterwards. You may be quite gifted, brother, but this potion is way too advanced; I'm staying until it's finished, whether you like it or not.
-Fine. Just shut up.
The silence of the dungeons was only broken by the bubbling of the thick liquid in the cauldron.
Mycroft knew Sherlock generally used that room that had fallen into disuse (or maybe had never been used, even) to practice very complex potions and spells, away from the judgmental eyes of the teachers. He generally didn't disturb him, but as his experimentations became more and more dangerous, Mycroft became increasingly concerned.
-The burner, Sherlock! For God's sake!
-Ok, ok. I got it.
Sherlock turned down the flame just before the potion boiled over.
-Thanks.
-And you should be literally pulverizing that, Sherlock; I wouldn't even call that crushed.
-I'm getting to it! God, Mycroft! Why don't you pulverize them then? Just sit on them; that shall do the trick.
Mycroft only rolled his eyes slightly at his brother's remark.
-Don't you ever get terribly tired of that kind of joke?
-Actually…
Sherlock's eyes left his potion for the first time since he had started brewing and set on his brother. Mycroft wasn't sure why, but every time Sherlock stared at him like that he found it extremely unnerving.
-I always found them quite entertaining. I know it's not the highest form of humor, but you should see your face sometime; it is quite hilarious. Still, I have to admit I haven't been getting the expected reaction lately… Which doesn't make sense, because you have been putting on weight nonstop for a few weeks now, one would anticipate an immediately proportional drop in self-esteem, and hence a rise in amusingness of the joke. So, what has changed?
Mycroft was at a loss for words. If Sherlock was able to deduce what was going on… Lord, he didn't know what he would do.
-Nothing has changed, Sherlock.
-Yes, yes it has.
Sherlock's face suddenly became disfigured with revulsion.
-Are you and Lestrade…? Are you two sleeping together?
But at the precise moment Mycroft's face was turning crimson red, and by the grace of a higher power, the potion spit up and covered Sherlock's right cheek. With the back of his hand he removed what he could of the thick, yellowish solution off his face.
-Never mind. I don't actually want to know. I really, really don't want to know.
He empathised each word by accompanying every one of them with a shake of his hand, as he tried to get rid of the rests of potion on it.
Mycroft nodded.
-Well, you appear to have everything under control, brother. You still have a little… oh, there, you got it. Go to the infirmary if it starts to itch. But really, you should be fine. I, um… I'll be on my way.
He was out of the room faster than light. Sherlock would forget all about that dire conversation in a couple of days. Ok, weeks, more likely. Mycroft really hoped he didn't ever bring that up again, but he trusted Sherlock would be too overcome by disgust to even try. Yes, he would delete that information as soon as his brain allowed him to do so. Thank God.