This came up during tutoring. Alexander learned what a perfect number is, a positive integer equal to the sum of its divisors (including one and excluding itself). Six is a perfect number: 6 = 1+2+3. Twenty-eight is a perfect number: 28 = 1+2+4+7+14.
Then we learned that all even perfect numbers are triangular numbers. That is when Alexander reminded me, no, chastised me, about Pascal's triangle. Which boy forgets nothing?
artwork courtesy of Wikipedia and David Tung on Quora