Burning firewood that isn't properly seasoned can lead to inefficient burning, pollution and smoke. One way to tell if your firewood is seasoned -- meaning dry enough -- is to simply bang two pieces together. If it sounds hollow, it's likely seasoned. If the wood sounds solid and dull, it's wet and should not be used. Other indicators include the color of the wood. If it's gray or yellow, it's had time to dry out. And the final test: If you're burning firewood and hear a hissing sound, it means the firewood is wet and should be left to season for a few months more.