second law of thermodynamics

The second law of thermodynamics says that, left to itself, the world always slides toward more disorder, never less. Energy spreads out, heat flows from hot things to cold things, and the universe never tidies itself back up on its own.

You see it every day. A hot cup of coffee always cools down to room temperature — it never sits there and spontaneously gets hotter while the room gets colder. Heat moves from warm to cool on its own, but never the reverse unless you spend energy (like a fridge running on electricity) to force it.

Scientists measure this 'spread-out-ness' with a quantity called entropy. In an isolated system — one nothing gets in or out of — entropy can stay the same or grow, but it can never shrink. That one-way rule is also why no engine can be perfect: some energy always escapes as waste heat, and you never get it all back as useful work.