how does concrete hold posts in the one spot in the ground

In the absence of a stupid question category, I will post this here. My question, on the surface, seems ridiculous. we all know that concrete hardens and therefore holds the post firmly. but what stops the concrete and the post moving in the ground. even when the job is large and reo is used, say foundations: what stops the whole lot moving. If your ground is hard and doesn't hold water easily, its obviously less likely to shift. but what about areas where the ground isn't so hard?


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