What you're doing there is not a contract, it's an invitation to bargain (aka invitation to treat). You make an offer, the counterparty can accept, reject, or negotiate, and once you have reached mutually agreeable terms you conduct an exchange and your business is over. There may be an implied contract (eg that the pond has not since dried up) but not necessarily.

A contract, on the other hand, is a forward looking document. It's based on the idea that you'll reach an agreement and something of value will be exchanged to seal the deal (which is why you hear of contracts for $1), but that the exchange will be incomplete until some time later.

For example, if you buy a concert ticket, your transaction with the vendor ends as soon as you receive the ticket, but the ticket itself is (typically) a kind of contract between the holder and the promoter, who promises to stage the event and provide admission to all ticket holders or refund the face value.