A flush toilet or water closet WC is a toilet that disposes of the waste by using water to flush it through a drainpipe to another location. Modern toilets incorporate an 'S' bend; this trap creates a water seal which remains filled with water between flushing, thus providing a hygienic barrier by preventing sewer gases from passing up the drainpipe. During flushing the S bend also provides siphon action which helps accelerate the flushing process. Water filling up the bowl creates a high pressure area which forces the water past the S bend. At the S bend when water starts to move it creates a vacuum that pulls the water and waste out of the toilet. When no more water is left then the air stops the siphon or vacuum process. At that point the water that is going into the bowl continues to fill up the bowl to equalise the bowl and the S bend. This ends the cycle of one flush. However, since this type of toilet does not generally handle waste on site, separate waste treatment systems must be built.