A valve for discharging a fluid, comprising: a valve body at least partially defining a fluid chamber; and a valve stem extending into the fluid chamber, the valve stem comprising an outlet port for transfer, in use, of fluid from the fluid chamber into the valve stem; the valve stem being slidably movable relative to the valve body from: i) a non-dispensing position in which the outlet port is out of communication with the fluid chamber; to ii) a dispensing position in which the outlet port is in fluid communication with the fluid chamber so as to permit transfer of the fluid from the fluid chamber into the valve stem; wherein the valve further comprises a locking member which is configured to prevent return of the valve stem into the non-dispensing position once the valve stem slides beyond a locking position.