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For a Contract to be valid, the offer it contains must be accepted (see: Acceptance of offer). If I lose my wallet, for example, and post a message in the newspaper that I will pay £100 for its safe return, then anyone who sees the message and returns the wallet is entitled to the reward. I will be in breach of promise if I refuse to stump up. However, if someone finds the wallet and returns it without seeing the message, I am not obliged to pay the reward. This is because the finder was unaware of the the offer and could not be said to have accepted it. Therefore there is no binding agreement. The case of Carlill v Carbolic doesn't help the finder (see: Carlill V Carbolic Smoke Ball Co (1893)) here; the finder could not later claim that his behaviour was compatible with acceptance by conduct, because his conduct was not based on knowledge of the offer. The motto of this story, of course, is that if you find a lost wallet or watch, check to see if a reward is offered before you return it, as you don't have a case afterwards.