Break point.

Unbreakable?

Put a glass cup at the centre of a round table, and each day, push it by a centimetre. In any direction, without ever looking at the position of the cup. After some time, on some fateful day, I am certain that the glass will fall off the edge, and it will break. How hard? Only God knows, but maybe it'll still be fixable.

Maybe, it won't. So, I suggest you not to push it.