The air lock phenomenon can be used in a number of useful ways. The adjacent diagram shows an 'S' trap. This has the properties a) that liquid can flow from top (1) to bottom (4) unhindered and b) that gas cannot flow through the trap unless it has enough extra pressure to overcome the liquid head of the trap. This is usually about 75 to 100 mm of water and prevents foul smelling air coming back from water drainage systems via connections to toilets, sinks and so on. 'S' traps work well unless the drainage water has sand in it – which then collects in the 'U' part of the 'S'.