Most building water supply systems contain a connection where the water piping system for the potable water supply crosses with a non-potable water supply. All of the building’s water needs—including fire sprinklers, toilets, sinks, and water fountains—all flow in from the city’s main water supply. Backflow occurs when water pressure outside the building drops and non-potable water is able to flow outward, back into the potable water supply, contaminating that water source.