user: what is petrified wood

assistant: Petrified wood is one of fossils, it is a plant which the inner part has been replaced by stone by compressions and impressions through millions of years

user: From the passage note down the various themes on which Harry Potter series is based upon. List the results in comma separated format.

assistant: death, prejudice, corruption, madness

user: As a child, what singer held the longest note ever on Star Search?

assistant: Usher