Why are 6 and 28 called perfect numbers?

6 and 28 are called perfect numbers because they are equal to the sum of their proper divisors (excluding themselves). For 6, the divisors are 1, 2, and 3, which add up to 6. For 28, the divisors are 1, 2, 4, 7, and 14, which sum to 28. This unique property defines perfect numbers in number theory.