Why should a vehicle be in gear or have its parking brake engaged when using a jack?

Having a vehicle in gear or engaging its parking brake while using a jack is crucial for safety reasons. This precaution is taken to prevent the vehicle from rolling or shifting while it is elevated. When a vehicle is lifted, the center of gravity changes, which can make it unstable. If the vehicle were to roll while being supported only by a jack, it could cause significant injury to anyone nearby and result in damage to the vehicle itself. By ensuring the vehicle is either in gear (for manual transmissions) or has the parking brake engaged, you can minimize the risk of unexpected movement, making the lifting process safer for both the operator and anyone in the vicinity.