With summer around the corner, the Environmental Protection Agency has tips on how to properly fill up your car's gas tank, especially during hot, sunny days. Try to fill up when it's coolest -- like in the morning or night -- since any accidental spillage of gasoline will evaporate right into the atmosphere when it's warmer outside. When gasoline evaporates, it becomes a vapor that contributes to bad ozone and toxic air pollution. And don't top off the tank after the gas pump automatically stops. Your car needs extra room in the tank to allow the gasoline to expand. If you top if off, extra gas will escape into your car's vapor collection system, which wastes both money and fuel.