Field
Thermodynamics
Heat, disorder, and the one law with a direction in it.
The speeds in a still room
The air in a quiet room is not still. Every molecule in it is moving at hundreds of metres per second, and temperature is a single number summarising an entire distribution.
Entropy is a count, and the arrow of time is arithmetic
Nothing in mechanics prefers a direction. Entropy is not a force pushing things toward disorder — it is the observation that some outcomes have vastly more ways of happening than others.
The ceiling on every engine, set before it was designed
There is a maximum efficiency no heat engine can exceed, and it depends on nothing but two temperatures. Not the fuel, not the working substance, not the cleverness of the engineer.