A day of awkward conversations and laughing

In science today we watched a video about the human reproductive system. It was as unawkward as these videos go, but that didn't help much. Afterwards, we had a discussion about the video.
Somehow we got onto the topic of types of diseases or problems passed on from the mother to the child. The substitute (we'll call him Mr. G) said, "If you were walking in the park and all of a sudden your leg gets chopped off..."
Before he could finish his sentence, the class burst out laughing. Somebody stood up and started walking around, then suddenly collapsed to the floor, hiding one leg behind his body. From where I was, it actually looked like his leg disappeared. Mr. G told him to go back to his seat and tried again: "If you were walking in the park and all of a sudden your leg falls off, your baby would still have two legs."
This wasn't much better. In fact, the image of a leg suddenly falling off was funnier than a leg being chopped off for no reason. Mr. G noticed this and tried one more time.
"If you somehow lost a leg, your baby would still have two legs."
I'm not sure whether anybody other than me heard this, because everybody was still busy trying to think of scenarios in which one's leg would fall off while walking in the park. Mr. G noticed this as well, and tried to change the subject. We ended up talking about Siamese twins.
"Did you hear about those Siamese twins that were connected at the head?" one student asked. "What happened to them?"
"I'm pretty sure they died," Mr. G responded.
"They died? Why? They were talking about them a long time ago when they were born and the news said that they were healthy..."
Mr. G looked at the student strangely. "Why did they die?"
"Yes."
"Because they shared a head!"
"What?" asked the student. Now everybody was confused. "They shared a head?" Suddenly, the student seemed to realize something. What it was, I'm not sure, but he remembered what he was talking about. "Oh, of course they died."
We continued talking about Siamese twins, but the focus changed to separating them. "Well," Mr. G said, "sometimes it's safe, but other times it isn't. Like, for example, if they were connected at the chest and shared internal organs."
I heard someone say "You get the lungs, liver, and stomach; you get the intestines and the heart."

Later that day, in English class, someone made a joke. It wasn't very funny, but I started laughing anyway. And, as much as I tried, I couldn't stop.
First, people weren't really paying attention to me, but as I laughed for longer and longer, people started to watch. My teacher, after locating the source of the disturbance, told me to get a drink of water. I did, and I seemed to get better. For a few minutes.
I was editing my essay and I came to a sentence that made no sense. I started laughing before I could even process the information. This time was even worse. I couldn't stop laughing for even a second. I started to rock back and forth in my chair. Then, not long later, I fell on the ground and rolled around. I couldn't help it.
Again, my teacher ordered me to get a drink. I pushed myself up onto my feet, but no sooner had I reached a kneeling position then I fell over again. I couldn't keep my balance. I started coughing and laughing and laughing and coughing.
At that point, I realized that half the class had dropped what they were doing to stare at me, and I awkwardly inched out of the classroom and bolted to get a drink. I somehow managed to stop laughing before I choked on the water.

-Marianne