Contingent contract enforceability depends on occurrence of the specified future event; impossibility renders the contract void. Contracts conditioned on an uncertain future event are not enforceable until the event occurs; the happening of the event is a condition precedent to enforcement and if the event becomes impossible the contract is void for impossibility of the condition. Examples include promises contingent on a promisor surviving a third person, on a third party's refusal to buy, or on a specified marriage that does not occur.