Expert stonemasons, or stone sculptors, know exactly where to hit a stone. They know exactly how much pressure to apply to make the stone break or chip the way they want it to. They take off all of the pieces that don't belong, until they reach the final piece hiding inside of a bigger block of stone.
God does the same with us. He takes pieces off of us. Sometimes in large chunks, sometimes in tiny chips, until we reach just the right size and shape. Trials and tribulations are the hammer and chisel with which God shapes us.
We can either choose to resist his final plans for us, and come out as a deformed and worthless lump of rock... or we can work with him, and watch as his guiding hands shape us into the men and women we were meant to become.