What kind of book could be worth more than a mansion, a private island, or even a fleet of luxury cars? Meet the Codex Leicester: a 72-page notebook that was once owned by none other than Leonardo da Vinci himself! This isn’t just any notebook, though. It’s filled with da Vinci’s sketches, musings, and genius ideas, written in his famous mirror-script (backwards, because why not?).

Back in 1994, this treasure of human creativity went up for auction, and guess who decided to make it part of his personal library? Bill Gates, the tech billionaire himself. Gates bought the Codex Leicester for a staggering $30.8 million, which, with inflation, is roughly $63 million in 2025! 

The Codex is like peeking into da Vinci’s brain, filled with notes on topics like astronomy, fossils, water flow, and even why the moon shines. Gates even took the notebook on a "digital tour" in the late '90s, making its pages accessible to the public for the first time in centuries, proving that some ideas are simply too valuable to keep locked away.