* For the first spot on the shelf, you have 13 choices.
* For the second spot, you have 12 books left, so 12 choices.
* For the third spot, you have 11 choices, and so on.
This continues until you reach the last book, where you only have one option left.
To find the total number of arrangements, you multiply the number of choices for each position:
13 * 12 * 11 * 10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1
This is called a factorial and is written as 13!
Using a calculator, 13! = 6,227,020,800
So there are 6,227,020,800 ways to arrange 13 books.