return adoptionInstance.adopt(petId, {from: account, value: 1000000000000000000});

function adopt(uint petId) public payable returns (uint) {
        require(petId >= 0 && petId <= 15);
        adopters[petId] = msg.sender; // msg.sender is the address of the person or smart contract instantiation it.
        return petId;
        // return as a confirmation that the petId x has been adopted and its adopter is stored in an array
    }