Most states require tires to be replaced when they have worn down to 1/16-inch (1.5 mm) of remaining tire depth. Tires sold in North America are required to have "wear bars" molded into them to make it easy to see when tire replacement is legally required. However, if you'll be driving in the rain, you should change your tires when there is 1/8-inch (3 mm) of tread left. Otherwise, water may not escape from under your tires fast enough and you risk hydroplaning, a dangerous situation in which your car loses traction and literally floats on the water. Stick an American quarter between the treads in several places. If part of Washington's head is always covered, you have enough tread to drive in the rain. If you drive in snow, you'll need at least 3/16-inch (5 mm) of tread to get adequate traction. Stick an American penny between the treads. If the top of the Lincoln Memorial is always covered, you're ready for winter driving.