A gallon of paint is heavier than a gallon of water. Mostly anything that mixes a solid into the water will be heavier than water. Most homes have a can of Spackle , which is a common liquid for patching drywall and it is almost twice as heavy as water. Of course,mercury is the heaviest liquid, but isn't common around the house anymore. But if you have a mercury switch for turning on your furnace, then that would be the heaviest common liquid by far. Which, I assume, discounts mercury - oil is lighter than water, so two choices would be sugar water or salt water.