A few questions

• You keep hitting the desk – you heat it
• A lamp shining on the desk ... heats it. Turn it off – the desk cools
• Push something along ... it stops. “Friction,” right?   What does this mean ?
• Drop the keys ... they hit the desk and stop; where did the kinetic energy go ?
• Incandescent bulb : ~ 10% efficiency.   The rest ?   Where is the rest of the energy ?

Ever thought about it ?   Why / how (exactly) does this happen?

Where does the heat ‘come from’ ?   ( ... what is heat ? )   Where does the energy go ?

Development of the molecular / atomic model for the structure of matter provided a framework.