Condensation occurs on plastic water bottles due to moisture in the air coming into contact with the cooler surface of the bottle. When a bottle filled with cold water is exposed to warm, humid air, the temperature difference causes water vapor to condense into liquid droplets on the exterior. This phenomenon is distinct from leakage, as the bottle remains intact and does not allow liquid to pass through. A practical experiment is suggested: using warm water in the bottle will prevent condensation, demonstrating the effect of temperature on moisture. Understanding condensation is essential for grasping basic thermodynamic principles.