The concept that economists call "value added" is important to understand exploitation. Marx's economic theory goes something like this, making up some fictitious numbers for an example. Suppose the capitalist invests $10 in some raw materials and the wear-out of tools, plus $15 on labor power. Suppose that labor converts those materials into a product that sells for $100. As Marx saw it, the labor power gave the work-in-process a value added of $90, because that labor changed the $10 materials into a $100 product. But the worker only got paid $15. The worker's wage is the fraction 15/90, or about 17 percent, of the wealth that he or she produced. Therefore, the source of the employer's profits is an extraction from the workers' wages. Every time the worker gets paid, the worker is robbed.
Marx gave names to all these terms. The $10 for materials and wearout of tools is called a "constant capital", because it adds only its own value to the product, just the $10. The labor power that the capitalist bought is called "variable capital" because an expansion occurs when "labor power", the ability to do work, which has a value of $15, becomes "labor", which is actual work performed, which has a value of $90. The expansion of variable capital occurs after the capitalist has bought it (the capitalist owns the use of the worker's mind and body), therefore the capitalist and not the worker receives the profit.
The process is called "exploitation", which means "use", because the capitalist uses the worker as what Marx called "an appendage to the machine." The capitalist considers the worker to be, not a human being, but merely an instrument to be used for its ability to function as variable capital and produce value added.


, although you know, as well as I know, that the workers would make a lot more and the boss a lot less.