Your muscle or your proteins in your body are like a brick wall. The amino acids are the bricks going in; protein synthesis is the bricks going into the wall. But at the other end, there's a process of... bad bricks, damaged bricks, old worn-out bricks being taken out of the wall. So, we're turning protein over constantly. And if you're performing exercise... then that protein turnover process is accelerated. So, you have to put more bricks into the pool to replace the ones that are coming out.