Write down a three-digit number whose digits are decreasing.
Then reverse the digits to create a new number, and subtract this number from the original number.
With the resulting number, add it to the reverse of itself.

The mystery of the number 1089.  Write this number down.

C > B > A Called number A B C (reverse) ------------ (subtract) Z   Y   X   X   Y   Z (reverse) ------------ (add up) 1   0   8   9 (answer)         3 > 2 > 1 1 2 3 (reverse) ----------- (subtract) 1   9   8   8   9   1 (reverse) ----------- (add up) 1   0   8   9 (answer)                                                                         9 > 7 > 5 5 7 9 (reverse)  ----------- (subtract) 3   9   6   6   9   3 (reverse) ------------ (add up) 1   0   8   9 (answer)         8 > 3 > 2 2 3 8 (reverse) ----------- (subtract) 5   9   4   4   9   5 (reverse) ----------- (add up)   (answer)

The mystery is this: first, take any three digit number,
where the first and last digits differ by two or more and reverse the number to produce a new one.
Then subtract the smaller from the larger producing another new number.
If you reverse this number and this time add the two,  
answer is always 1089 - Math is Fun.