Charlie, a poor musician, needs $1500 fast to pay his back rent and buy a food stand. Stan agreed to lend him the $1500, with Charlie's saxophone as collateral. They put their agreement in writing and sign it. Charlie keeps possession of the saxophone so he can play gigs to make money and eventually pay Stan back. Explain: Does Stan have an enforceable security interest? If so, how can Stan let other creditors know of his interest in the saxophone? If not, how can he get a security interest in the saxophone?