This is the easiest fix ever. Pour about a gallon of water into the drain and you are done. This has to be done often due to evaporation. If you want to prevent evaporation there is a trick that is simple. After you pour 1 gallon of water then follow it with 1/2 cup of vegetable oil down the floor drain, the oil sits on top of the water constantly making a seal so the water can not evaporate and always making a trap seal.