Just 2 questions

I need help with a Economics question. All explanations and answers will be used to help me learn.

Capital Volume 1 Karl Marx

Introduction to Marxist Analysis Assignment #2

On page 139 Marx states “Everyone knows, if nothing else, that commodities have a common value-form which contrasts in the most striking manner with the motley natural forms of their use values. I refer to the money form. Now, however, we have to perform a task never even attempted by bourgeois economics. That is, we have to show the origin of this money form, we have to trace the development of the expression of value contained in the value relation of commodities from its simplest, almost imperceptible outline to the dazzling money form.”

Then at the bottom of page 186 Marx writes “The difficulty lies not in comprehending that money is a commodity, but in discovering how, why and by what means a commodity becomes money.”

How does Marx explain the why a commodity becomes money?