I gather all kinds of facts this way- or should I call them opinions. Too many sources to remember.. but generally I do.
I was showing some students the nature of multiplying by the number 9, and the results..
9 X 2 = 18, 1 +8 = 9
9X 4 = 36, 3 +6 = 9
9 X 32 = 288, 2 + 8 +8 = 18, 1 + 8 =9.
I cited my source. It was from Sesame Street.
Of course it is far more interesting when Muppets are singing and dancing about numbers.. heh