Why do we say "Believe it or not"?

Meaning:

It may sound strange, but it's true

Background:

This expression was made famous by American artist Robert LeRoy Ripley whose 'Believe It or Not' cartoons were syndicated in many American newspapers starting in 1918. However, it seems the expressions had been around for a few years longer, possibly into the 19th century. Robert Ripley died on an island in Long Island Sound in 1949; the island was named Bion, an acronym of Believe It or Not.

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