The key difference between a standard sprayer and a foamer is the addition of an air intake system and a mixing chamber.

1. The Dip Tube: Like a standard sprayer, it draws liquid from the bottle.

2. The Air Intake Valve: This is the crucial extra part. As you create pressure and vacuum with the trigger, this valve draws air into the pump mechanism.

3. The Mixing Chamber: The drawn liquid and air are forced together under pressure into a special chamber, typically filled with a mesh screen or porous filter.

4. Foam Creation: This mesh screen is the heart of the foamer. It breaks down the liquid-air mixture into tiny, uniform bubbles, creating a stable, dense foam.

5. Dispensing: The fully formed foam is then expelled through the nozzle.

In short: Liquid + Air + Mixing Mesh = Foam.