1. The problem asks if 21 is a prime number. 2. A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. 3. To check if 21 is prime, we test divisibility by prime numbers less than or equal to $\sqrt{21}$. 4. Since $\sqrt{21} \approx 4.58$, we check divisibility by 2, 3, and 5. 5. 21 is not divisible by 2 because it is odd. 6. Check divisibility by 3: $21 \div 3 = 7$, which is an integer. 7. Since 21 is divisible by 3, it has divisors other than 1 and itself. 8. Therefore, 21 is not a prime number.