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Some numbers are stubborn. Like certain people you may know, these numbers — called prime numbers — resist being placed in any sort of a box. Look at how Figure 1-4 depicts the number 13, for example.

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FIGURE 1-4: Unlucky 13, a prime example of a number that refuses to fit in a box.

Try as you may, you just can’t make a rectangle out of 13 objects. (That fact may be one reason why the number 13 got a bad reputation as unlucky.) Here are all the prime numbers less than 20:

As you can see, the list of prime numbers fills the gaps left by the composite numbers (see the preceding section). Therefore, every counting number is either prime or composite. The only exception is the number 1, which is neither prime nor composite. In Chapter 8, I give you a lot more information about prime numbers and show you how to decompose a number — that is, break down a composite number into its prime factors.