So I'm sitting at the table the other day explaining factors to the primary-age little one, including some of the little "tricks" one can use to determine if a number is a factor, such as that if you add up all the digits of a multiple of 3, they will sum to 3, 6, or 9 (given enough iterations). A few minutes later, I ask, is 4 a factor of 72?

LO says "yes" immediately (even though LO does not have the multiplication facts memorized, and still does repeated addition to divide multi-digit numbers by 2). Surprised, I ask how LO knows this. Answer: the tens place is an odd number, and the ones place is a 2. I had to think about that for a minute.

I don't believe I had ever observed this, and, though math is not my personal strength, I am considered to be reasonably decent at math (I have a graduate degree in a STEM field). To top it off, the next child up quickly demonstrated that this is true for all whole numbers.
...pronounced like the long vowel and first letter of the alphabet...