Explanation: There are a few important things to keep in mind when changing a tire, even if you've changed a dozen or more in your lifetime. First: it is essential to put a wheel chock or a large rock behind (if the car is facing uphill) or in front of (if the car is facing downhill) the wheel diagonally opposed to the one you are changing. This will prevent rolling when the car is on a jack. Second: the valve where you add air to the tire should always face outwards – remembering this will prevent confusion if you can't tell which way to put the new tire on. And third: make sure to tighten the lug nuts gradually, alternating from one lug nut to the lug nut across from it, in a star pattern. Never tighten adjacent nuts consecutively. This will assure that the tire is aligned properly. Diesel technicians are often expected to perform this kind of operation under extremely adverse conditions – rain, sleet or snow on a busy mountain highway, for example, can make changing a flat tire on a semi-truck quite a challenge.