Professor Binns paused again,

"For a few years, the founders worked in harmony together, seeking
out youngsters who showed signs of magic and bringing them to the
castle to be educated. But then disagreements sprang up between
them. A rift began to grow between Slytherin and the others. Slytherin
wished to be more selective about the students admitted to Hogwarts. He
believed that magical learning should be kept within all-magic families.
He disliked taking students of Muggle parentage, believing them to be
untrustworthy. After a while, there was a serious argument on the
subject between Slytherin and Gryffindor, and Slytherin left the
school."

Professor Binns paused again, pursing his lips, looking like a wrinkled
old tortoise.

"Reliable historical sources tell us this much," he said. "But these
honest facts have been obscured by the fanciful legend of the
Chamber of Secrets. The story goes that Slytherin had built a
hidden chamber in the castle, of which the other founders knew
nothing.

"Slytherin, according to the legend, sealed the Chamber of Secrets
so that none would be able to open it until his own true heir arrived at
the school. The heir alone would be able to unseal the Chamber of
Secrets, unleash the horror within, and use it to purge the school of
all who were unworthy to study magic."

There was silence as he finished telling the story, but it wasn't the
usual, sleepy silence that filled Professor Binns's classes. There was
unease in the air as everyone continued to watch him, hoping for
more. Professor Binns looked faintly annoyed.

"The whole thing is arrant nonsense, of course," he said. "Naturally,
the school has been searched for evidence of such a chamber, many
times, by the most learned witches and wizards. It does not exist. A
tale told to frighten the gullible."

Hermione's hand was back in the air.