Experiments you can run yourself.
The first law of thermodynamics: how heat and work change the internal energy of a gas. An interactive simulation of gas in a cylinder and of thermodynamic processes.
The second law of thermodynamics: why heat flows on its own from a hotter body to a colder one and why entropy increases. Interactive simulation.
Specific heat: how much energy it takes to warm a substance, and why water heats up slowly. An interactive simulation of Q = mcΔT and of heat balance.
Isothermal, isobaric, isochoric — p-V diagrams and the quantity that stays constant.
η=1−T_c/T_h. The greater the temperature difference, the higher the efficiency.
Close to absolute zero, atoms "merge" into one shared quantum state (Nobel Prize 2001).