1. The problem asks why the difference between 10 and 1 is 9. 2. The subtraction operation is defined as finding how much one number exceeds another. 3. The formula for subtraction is $a - b = c$, where $a$ is the minuend, $b$ is the subtrahend, and $c$ is the difference. 4. Here, $a = 10$ and $b = 1$, so we calculate $10 - 1$. 5. Subtracting 1 from 10 means counting how many units remain after removing 1 unit from 10 units. 6. This results in $10 - 1 = 9$ because removing one unit from ten leaves nine units. 7. Therefore, the difference is 9 because subtraction measures the amount left after taking away the subtrahend from the minuend.