If you live in a reaaaaaaaallllly cold place, then there's a slight chance that your gas will freeze, if there's a tiny amount in the tank. However, this is not the main problem, and it's not why we say to keep the tank half full. When there is less than half a tank of gas, condensation is more likely to build up, which can then get into the gas lines and freeze them up. With frozen gas lines, no gas can get to you engine, and the car will not be able to run. Plus, more gas = more weight = better traction.