[b][Caution: May include boring math stuff.][/b] Perhaps you're the person repeating "9 + 10 = 21" on endless loop. Or maybe you're the reasonable human who's gotten tired of that nonsense. Either way, here's a math trick my friend and I figured out to actually make 9 + 10 = 21. As you may know, we widely use base 10 mathematics. Our counting system goes as follows: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11... And so on. We have 10 unique digits: 0-9. This is known as a radix, or base of 10. The Babylonian civilization used base 60, with 60 unique digits including 0. In base 10, 9 + 10 = 19. But what if we wrote the answer in a different base to make it 21? We can do something interesting in base 9. 9 (in base 10) = 10 (in base 9) 10 (in base 10) = 11 (in base 9) 10 + 11 = 21 in base 9 just like base 10, so what we end up with is this: 9 + 10 (in base 10) = 21 (in base 9) 9 + 10 = 21 Thank you for reading.