Fun Math Facts.

I have to say my fav fact is the multiplication by 11.
Let b1, b2, .... , bn be the digits of a number, then

b1*11 = b_1b_1
b1b2*11=b1(b1+b2)b2
b1b2b3*11=b1(b1+b2)(b2+b3)b3
[...]
b1b2...bn*11=b1(b2+b3)....(bn-1 + bn)bn

This "fun fact" could be proved by induction.