The blood donation centre in the city hospital keeps a short notice taped beside its door, and the whole notice is two lines long.
«To give blood you must weigh at least 50 kilograms, and you must be aged between 18 and 60.»
The nurse who works the front desk says the same sentence forty times a day, because people read one line and stop. A strong seventeen-year-old who weighs 62 kilograms meets the weight rule easily and is still turned away. A woman of thirty who weighs 48 kilograms is exactly the right age and is also turned away — kindly, and with an invitation to come back.
Neither of them has been treated unfairly. The notice does not offer two ways of qualifying; it sets two conditions that must hold at the same time.
Both rules exist for a reason the nurse is happy to explain. The weight limit protects the donor: the volume taken is fixed, so a lighter body loses a larger share of its blood. The age range protects the donor too, at both ends.
A woman of thirty who weighs 55 kilograms meets both, and gives blood in eleven minutes.
If you drew the two rules on a chart — weight along one side, age along the other — the permitted area would be a neat rectangle, and every person turned away that morning would be a dot just outside one of its edges.