Jules agrees to pay Dr. Smekyll a million dollars and Smekyll agrees to invent an automobile engine that runs on air rather than gasoline. Why would this contract not be legally binding? What could Jules and Smekyll do to make sure that their contract is enforceable?

The contract between Jules and Dr. Smekyll may not be legally binding due to an issue of mutuality of consideration. In a valid contract, both parties must provide something of value (consideration) to each other. In this case, Jules agrees to pay a million dollars, but Dr. Smekyll's promise seems to be merely an illusory promise, as it depends on inventing something that may or may not be achievable.

To make sure their contract is enforceable, Jules and Smekyll should consider the following:

1. Clear and specific terms: Ensure that the terms of the contract are clearly defined, leaving no room for ambiguity. For example, instead of simply stating that the engine should run on air, they can define specific performance metrics that the engine must meet.

2. Meeting of the minds: It's important that both parties have a mutual understanding of their obligations and expectations. They should openly discuss and agree upon the scope, timeline, and other relevant details surrounding the invention of the engine.

3. Promise supported by consideration: To address the issue of mutuality, Dr. Smekyll could add a clause to the contract to demonstrate a commitment and provide consideration. For example, he could agree to undergo regular progress updates, submit reports, or invest his time and resources to actively pursue the invention.

4. Contingencies and termination clauses: Include provisions that address what happens if certain circumstances arise or if either party wants to terminate the contract. These clauses will provide clarity on how to handle unforeseen events without necessarily invalidating the entire agreement.

5. Seek legal advice: Both Jules and Smekyll can consult with a lawyer experienced in contract law to ensure their agreement is legally enforceable. A lawyer can help identify potential pitfalls, draft the contract properly, and provide guidance on relevant laws and regulations that may impact the contract.

Remember, I am an AI language model, so it's always crucial to consult a legal professional to ensure the specific context and jurisdiction are properly considered when creating a contract.