There is a box with 99 white balls and 101 black balls in it. And you have a lot of white and black balls outside the box. 

You pick two balls at random from the box. If they are a black and a white, you throw the black ball out and put the white ball back in the box. If however, you got two whites or two blacks then you throw both of them out and instead put back a black one from the pile outside.

Now, you keep doing this till you have one ball left in the box.

What color is the ball?