Very early in our childhood we are taught that you can't eat your cake and have it.
And this is only putting in another way the fact that you cannot use the same resourses to produce two separate things at the same time.
If you want to grow cabbages on a bit of land you cannot also use it for a tennis court; if you spend a couple of hours in a cinema you cannot also use them for a digging the garden or painting a picture or mending the boots.
You have to choose which of the many possible uses to which you could put the same materials or the same time is the one you prefer.