If you don't know what a ballcock is, just look for the little ball floating in the back of your toilet. Among the variety of ballcocks out there, one popular option is the anti-siphon ballcock. This product cannot be found in many old toilets, but it is vital to the protection of your water supply as a whole.
Without an Anti-Siphon Ballcock
If you have a time when the pressure of your water is low, you run the risk of siphoning. What that means is that there is a chance for the water in your toilet to get sucked up into the other pipes that control your water supply. Your drinking water and bathing water could be contaminated very easily if, for instance, low pressure resulted from a firetruck using a lot of water in your area.
The Anti-Siphon Solution
As a way to ward off this siphoning effect, you can get an anti-siphoning ballcock. This device will help maintain the pressure of the water so that your toilet remains unaffected by any actual changes that may be occurring. Your water can remain waste-free if you install one of these in your toilet.