You cannot get blood out of a stone.

The proverb describes the futility of trying to obtain something from a person or situation that cannot supply it. More pressure cannot create what is absent.

It distinguishes inability from refusal: a person may fail to meet a demand because they lack the means, not because they are unwilling to help.

Use it when someone keeps pressing for a contribution or result that the person being asked cannot provide. It can help shift the discussion from demanding more effort to recognizing a practical limit.