A stirrer having a motor; a hollow shaft that is drivable via the motor and is provided with at least one additive outlet opening, via which an additive passed through the hollow shaft can be discharged; and a rotor arranged on the hollow shaft and having rotor blades, characterized in that a second rotor having rotor blades is provided on the hollow shaft at a distance from the first rotor, and in that the at least one additive outlet opening is provided between the two rotors, wherein the rotors are designed and drivable such that, during operation, a negative pressure and a centrifugal force are generated in the intermediate space defined between the rotors.