The Pool Sentry is used to maintain the water level of a pool, spa or pond automatically and unattended. When The Pool Sentry is removed from the box, it is easily assembled by snapping together two parts and threading in the valve assembly. This requires no tools. It comes with a heavy plastic, zip-lock bag that is filled with rocks or pea gravel and used as a weight to hold the unit on the deck. The bag is placed inside the cavity by removing the top cover. It is then attached to a garden hose and the water is turned on. The water stays on at all times when The Pool Sentry is in the pool. As the float drops, the water turns on. When the float reaches its highest point, the water shuts off. It works very similarly to the float valve in a toilet tank. The float can be adjusted to any desired level (usually a point halfway on the tile line or skimmer).