Second law of thermodynamics
The second law of thermodynamics is one of the most important physical laws. It states that energy in the Universe tends to spread out and get less organized over time. This means that any place with lots of energy will usually send some of its energy to nearby areas with less energy until the amount of energy everywhere is even. For example, if you have a hot pan of food on the stove and just the pan is hot, the hot pan will cool down by sending some of its heat to the colder air around it until eventually everything is at the same temperature.
Related topics others have asked about:
Clausius–Duhem inequality,
First law of thermodynamics,
Fluctuation theorem,
Heat death of the universe,
History of thermodynamics,
Jarzynski equality,
Laws of thermodynamics,
Maximum entropy thermodynamics,
Quantum thermodynamics,
Reflections on the Motive Power of Fire,
Relativistic heat conduction,
Thermal diode