What is the easiest way to square a two digit number, without pen or paper?

Let us take 'ab' as the two digit number:

So,  (ab)^2=(a^2)(b^2) +20*a*b

For example let us square36:

Answer is (3^2)(6^2)+20*3*6=936+360=1296

Similarly for69

Answer is (6^2)(9^2)+20*6*9=3681+1080=4761