If you have an old toilet, you can save water with an easy trick. Take an empty two-liter soda bottle, wash it out, fill it with water, screw the lid on tightly and set it into your toilet tank, as far away from the flapper valve as possible. This prevents two liters of water from being used every time you flush. Will it leave enough water for a proper flush? A new low-flow toilet uses 1.6 gallons per flush. Older toilets use up to seven gallons a flush. Two liters is only half a gallon, so there’s still plenty of water left.