The supply curve is derived from the marginal cost of production, which is the additional cost of an additional unit. In general, the marginal cost is increasing with production, which causes the upward sloping supply curve. This increasing marginal cost exists because a firm has a fixed amount of capital in the short run, so the only way to increase production in the short run is by increasing the amount of labor used. In general, however, the marginal product of labor (the additional output of an additional unit of labor) is decreasing because the fixed amount of capital is spread out across greater amounts of labor. Because of this, it takes each additional unit of labor a longer period of time to produce an additional unit of output (all else constant), which makes each additional unit more and more expensive to produce.