I was tripping balls on 3 sheets of blotter, and I went on this complex train of thought about cartilage. The short version is something like:
"If you break cartilaginous tissue, it's very hard to get it to grow back together, which is why if someone has broken their nose it's usually pretty obvious. Sharks are made out of cartilage. What happens if you break a shark?"
Only it went on for about an hour and was (disturbingly) detailed. Also, on a similar 'cid trip I had a mate who was also tripping wearing a shirt that said "Mind the Gap." It spawned the idea that my mind was IN the gap, which lead to some really amusing exchanges.