Well casings serve several purposes in water wells. Together with the well screen it supports the borehole, protecting the well structure and pump. It also protects the ground water from contamination originating at the surface and in some cases plays an important role in the well construction process. There are four types of water well casings. They are classified according to function as surface casings, conductor casings, intermediate casing and pump housing casing. A well screen is a filtering device attached to the bottom of the well casing and serves as the water intake portion of the well. It is used to keep unconsolidated or semi-consolidated material from entering the well. The appropriate sizing of the screen slots are determined by the grain size of the material surrounding the bore hole. The correct size and spacing of the screen slots directly relates to the efficiency at which water can move into the well. A screen with relatively large slots may transmit a great amount of water, but may also allow sediment to enter the well. Conversely, a screen with smaller slots may restrict the entrance of sediment into the well, but also may restrict the flow of water hence screen should be selected accordingly. Bailer is like a length of pipe with a one-way valve in the bottom. The bailer is lowered into the well until it fills with water and sediment. It is then pulled to the surface and emptied. Water from the aquifer will then flow towards the well and bring in more drilling fluid. A bailer's up and down motion causes a surging action which will develop the area around the screen. The heavier and wider the bailer is, the better it will function because it will have more force to push water through the well screen.