Under every drain in your house, there's a trap that holds water and prevents sewer gas from flowing up into the air you breathe. But if you have a shower that's been abandoned since the kids moved out or a floor drain that never gets used, the water in the trap can eventually evaporate. That lets in stinky—or even dangerous—sewer gases. The solution is a few ounces of cooking oil. Dump a bucket of water down the drain to refill the trap. Then add the oil. The floating oil forms a seal over the water and prevents evaporation—without raising your cholesterol.