How Long Is a Piece of String: Geometry of Uncertain Mercy

Someone approaches you and asks for a piece of string. That's all they say. No context, no explanation, no qualifying details. Just: "Can I have a piece of string?" In that moment, you hold something more precarious than you might realize. You're standing at the intersection of mathematics, psychology, and potentially someone's survival. How do you answer? More importantly, how do you act?