Money becomes Capital

1.  Consider a value of money M that is
2. Exchanged for a commodity value C
3. Which is exchanged for money M'
4. Where M' is greater than M by S
5. We then have a circuit M - C - M' (M'=M+s)
6. For Marx, capital is any sort of value that goes through a circuit like this
7. S is what Marx calls "Surplus Value"

how is surplus value possible