Grease from the kitchen sink not only stops up the sink waste pipe, but it will often stop up the main sewer. When a pipe becomes choked with grease it cannot be forced out by pressure, or the use of potash or lye for the purpose of dissolving it. The only remedy in such a case is to cut the pipe and take out the grease. This is very expensive, and costs a great deal more than a grease trap that could have been placed on the sink when new, and would have prevented such trouble. Fig. 160 shows a device made specially for kitchon sinks in hotels and restaurants to prevent grease from getting into the waste pipes. It traps the pipe against air or sewer gas coming into the house, and is called a grease trap. In places where the grease trap is used it is a source of revenue as well as a prevention against the stopping of pipes by saving the grease, which is caught in the trap, and selling it for soft soap.