A foam lance is an accessory for a pressure washer, used to create and apply a thick, active foam to the surface being cleaned. It consists of the nozzle body and a container for the cleaning agent, such as car shampoo. When connected to the pressure washer gun, water flows through the nozzle, creating a vacuum that draws the cleaning agent from the container. This mixture of water and cleaning agent is then forced through a special mesh filter, where it is aerated and transformed into a dense foam. The resulting foam adheres well even to vertical surfaces, allowing the cleaning agent more time to act and break down dirt. This process is often used as a so-called "pre-wash" for cars, where the foam gently loosens dirt without the need for mechanical contact with a brush. On some models, the width of the foam jet and the ratio of the drawn-in cleaning agent can be adjusted, which affects its density.