Mycroft used to conduct experiments all the time. He loved testing things and experimenting what chemicals did what when mixed. When he was six he overheard someone mentioning something about someone's blood boiling. Mycroft was curious whether one's blood really could boil.
Since he couldn't get a human to agree to participate in the experiment (he had asked his mum, but she just smiled and said how nice it was that he was curious.) he recruited his hamster, Mr. Snuggles to help him. Mycroft decided that the best way to see whether Mr. Snuggles' blood could boil was to place him in the microwave like he had seen Mummy do with cold tea. Mr. Snuggles didn't survive the data collection.
Although Mycroft was distraught over the loss of Mr. Snuggles, he couldn't help but notice that his method of experimentation was a failure since Mr. Snuggles' blood wasn't even close to boiling.
The next day at school, Mycroft was called on to read a passage out of the book. The passage was about a man who got so angry that his blood began to boil. Mycroft, being as sensible as he was, saw this factual error and swiftly pointed it out to his teacher. He then began to share with his fascinated peers how he had come to his conclusion. He was just getting to the part where Mr. Snuggles was about to go into the microwave when he was whisked off to the headmaster's office.
Mummy and Daddy were called in. The headmaster told them that their son was disrupting the learning of his peers. On the way home, Mycroft insisted that he was "disrupting" anyone. He was simply telling the truth. It is at this point when Mycroft learns that telling the truth isn't always the right thing. From that point on Mycroft doesn't speak much in class for fear of being accused of "disrupting" the class.