linguistic:paraphrasing, stylistic:tone
Unilateral offer

Once an offeree has begun the performance in reliance on the unilateral offer, an implied obligation may apply to the offeror that the promise cannot be revoked. In the English case Errington v Wood, a father bought a house for his son and daughter-in-law to live in. He paid one-third of the price, and said to his son and daughter-in-law that if they paid the regular mortgage instalments for the remaining two-thirds of the price until the mortgage was paid off, then he would transfer the title of the house to them. Subsequently, the father died, and the mother claimed possession of the house. It was held that the father's promise to convey the title to the son and daughter-in-law was irrevocable once they had begun paying the regular mortgage instalments. No order of possession was therefore granted.