Say person X lost his valuable item and advertised a reward for the return of it. Person Y, who knew X personally, found the item before reading the ad, and returned it to X, who just thanked Y for that.
Later Y read the ad and demanded the reward. X refused.
Contract Law teaches us that it was perfectly legal for X to refuse as there was no contract. Which legal principles apply here?
P.S. It was a question in a test, to which I answered that the principle was past consideration as Y returned the item before reading the ad and so without expecting any reward. However, the tutor did not give any credit for my answer and explained that "X never agreed to pay Y anything. To accept an offer, you must be aware of it.".