A multiple of 11
Try 451 and its reverse 154: 451 - 154 = 297, which is 11 x 27. Write a three-digit number as 100a + 10b + c; its reverse is 100c + 10b + a, so the difference is 99(a - c) = 11 x 9 x (a - c) — always a multiple of 11, no matter the digits. (This is a three-digit fact: with an even count of digits, like 12 - 21, it can fail, so stick to odd-length numbers.)
