Why is the Sea Salty?

Hello.

Today, I want to write about "Why is the Sea Salty?", a level four book in the USBORNE FIRST READING series.
When I found this book, I thought the same thing with this title and I want to know about it.
I finished reading this and thought the story may be true for a moment.

A thief is one of the character of this book, I like the face because what he thinks appears his face.
I like the thief's look.

Please try to read it and get to answer to this title.

Thank you for reading.