It never belonged to the worker in the first place. This point is crucial to understanding the origin of surplus value. To be able to sell something it is first of all necessary to own it. The only commodity that the worker owns is his or her labour power, or capacity to work, which is sold in the wage contract to the employer. The employer, like every other purchaser of a commodity, is entitled to the use value that derives from its consumption.
Labour power is consumed by putting the labourer to work in the production of new commodities.
The employer puts the labourer to work, supplying the necessary raw materials, machinery, computer software, buildings and all the other means of production necessary for the task at hand. Those means of production have a value and they pass it on to the new commodities that are produced. In the case of raw materials, all of the value will be contained in new commodity. In the case of machinery, buildings, etc., which are used up over a longer a period of time, only a portion of the value they embody will be passed on and preserved in the new commodity. Thus the total value of the new commodity will be the value of the commodities used up to produce it, plus the additional labour which has been added by the worker in the production process, determined by the length of time the worker is employed.