Automatic 'closing' of an offer. Although no offer remains 'open' indefinitely, a properly communicated written offer is said to be open until it is revoked by the offeror before its acceptance or lapses due to (1) non-acceptance before its expiration date, (2) non-acceptance within a reasonable period (where no expiration date is specified), (3) the death either of the offeror or the offeree before its acceptance, or (4) having become illegal or impossible by the operation of law. An orally communicated offer lapses when the conversation ends, except where the continuation of the offer is clearly understood by both the offeror and the offeree.