Not long ago, saving water in a home was thought of primarily as a good way to save money.

It is still a good way to save significant amounts of money, but droughts and shrinking supplies of water in many areas have made careful use of it a practical necessity.

In most homes, the toilets should be the first target of a water-saving campaign. Each toilet uses three to seven gallons per flush, and if there is a leak in one of these devices, hundreds of gallons of water can be wasted daily. Even if there aren't any leaks, most toilets can be made to use less water.

Cutting the amount of water used in a toilet should begin with learning to understand how the mechanism works. Once the mechanism is understood, it's easy to check and correct leaks and the groundwork is laid for possible installation of repair parts or water-saving devices.

A toilet has two separate mechanical systems inside the tank, a water-supply system and a flush system. Either or both of the systems can develop problems, especially in older toilets, and leaking between the tank and bowl can start. These leaks are sometimes so slow and quiet that they go unnoticed.

Here's what to look for:

The Supply System: This is attached to the water-inlet pipe, which runs into the tank from the floor or wall, and includes an automatic valve that is actuated by a float -- often a hallow or light plastic ball on the end of a rod. The automatic valve is designed to shut off the flow of incoming water when the tank is refilled after a flush. Leaks in this system generally occur when something goes wrong with the valve itself or the float. If there is a leak in this system, you'll generally be able to see water running or seeping from the filled tank into the top of the overflow tube -- the hollow tube located near the middle of the tank.

If, after a flush, the tank fills and shuts off with the water level below the top of the overflow tube, there is usually no problem with the supply system.

If water does continue flowing into the top of the overflow tube after a flush, try these steps:

Make a slight downward bend in the rod holding the float ball so that the float hangs a bit lower in the tank. This should shut off the water sooner and halt the leak.

If the leaking continues, shut off the flow of water into the tank at the inlet valve, located under the tank. Unscrew the ball from the float rod and examine and shake it. If the ball is the hollow type and has water inside, or if it appears corroded or damaged, it must be replaced. Replacement balls are available at most hardware stores and home centers.

If neither of these two steps stops a leak into the overflow tube, the automatic valve itself is probably to blame. Valve repair parts are available, but the best remedy in this situation is to install a complete new supply-control system of the type made by Fluidmaster, sold at many home centers and hardware stores.

The Flush System: This simple system is actuated when the flush handle is depressed. A rod connected to the handle lifts a ball-stopper or similar device from an outlet at the bottom of the tank, and the water in the tank is released into the bowl.

Leaks in the flush system are sometimes easy to detect by the sound of water running or simply by removing the lid of the tank -- the water level in the tank will remain low long after a flush and the float ball is never raised high enough to close the inlet valve.

Flush-system leaks can often be slow and difficult to detect, however. One excellent method of spotting a slow leak is to put a little food dye, Easter-egg dye or other coloring material such as ink into the tank, then leave the toilet untouched for an hour or so. If the coloring shows up in the bowl, it means the colored water is seeping past the ball stopper and the flush system is leaking.