Milking is now performed almost exclusively by machine, though human technicians are still essential on most facilities The most common milking machine is called a cluster milker. This milker consists of four metal cups—one per teat—each lined with rubber or silicone. The cluster is attached to both a milk collection system and a pulsating vacuum system. When the vacuum is on, it pulls air from between the outer metal cup and the liner, drawing milk out of the teat. When the vacuum turns off, it gives the teat an opportunity to refill with milk. In most milking systems, a milking technician must attach the cluster to each cow, but the machine senses when the cow has been fully milked and drops off independently.