Most poetically answered by Flanders and Swann:

The First Law of Thermodymamics.

Heat is work and work is heat

The Second Law of Thermodymamics:

Heat cannot of itself pass from one body to a hotter body

Heat won't pass from a cooler to a hotter

You can try it if you like but you far better not-a

'Cos the cold in the cooler will get hotter as a rule-a

'Cos the hotter body's heat will pass to the cooler

Heat is work and work is heat and work is heat and heat is work

Heat will pass by conduction and

Heat will pass by convection and

Heat will pass by radiation

And that's a physical law