One of the things I'm best at is finding books. That are already on the shelf. In numerical (DDC) order. By author.
When a patron requests a book (at the counter), I look it up on my fancy-schmancy catalog. I'll write down the call number, author and title for them. So it might look like this:
How to tell if you have diabetes
Then, I'll tell them that it's 'half-way back to the right', or wherever in the stacks it's located. 80% of the time, they'll find it just fine.
The other 20, though, they'll come back up and tell me that they couldn't find it. I'll check the record and see when it came back. If it's been back for more than a week, it's either on the shelf or 'missing'. If it's less than a week, it's a gamble if it's downstairs waiting to be shelved, or on the shelf.
So I'll write down the info about the book on a slip for me, and I'll slide out from behind my desk and go hunting. Usually, it's right where it belongs. Exactly where it should be.
Patrons (adults and children) just aren't taught how to use the DDC (Dewey Decimal Classification), so a lot of them just don't know how it works. And I can understand that. Usually, once I show/find the book, they'll browse that section, and hopefully figure out how the system works.
It's actually really easy, and even before I became a librarian, I had many call numbers memorized. True crime, ghosts and animals were my go-to sections at my small-town library.