system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] How did Salazar Slytherin mark the entrance to the Chamber of Secrets with a form of indoor plumbing that wouldn't exist for 800 years?

Hogwarts was founded in the 10th century but sinks weren't invented until the 18th.
gpt
The entrance wasn't originally a sink. However, Hogwarts already had many forgotten underground tunnels and secret passages even 200 years ago. When they worked on modernizing the castle to use plumbing, they used some of these tunnels as a framework for the piping.

Now, how did the sink get *marked*: The entrance, most likely, is magically marked. And when the tunnels that led there were changed and (unintentionally) diverted to a sink, the magical marking transferred itself to the sink too, since it was the closest thing to an entrance there was.