An insurance deductible is the amount the policyholder pays before the insurer pays anything. Raising it lowers the monthly premium, sometimes sharply, which makes a high deductible look like an easy saving. The catch is that the saving is guaranteed and small while the exposure is uncertain and large. A household that could not find two thousand dollars at short notice has bought a cheaper policy that it cannot actually use when the moment arrives. The right deductible is therefore not the one that minimises the premium, but the largest amount the household could pay tomorrow without borrowing.