The second law of thermodynamics states that in any energy transfer or transformation, the total entropy of a closed system will either increase or remain constant over time. Entropy is a measure of disorder or randomness, meaning that natural processes tend to move towards a state of greater disorder. This law implies that energy conversions are not 100% efficient; some energy is always lost as heat, which increases the system's entropy. For example, when a car engine runs, it converts fuel into motion, but some energy is lost as heat, contributing to the overall increase in entropy.