ELI5: Explain Like I'm 5

First law of thermodynamics

The first law of thermodynamics is a scientific law that talks about energy. It says that energy can't be created or destroyed, only changed from one form to another. For example, when ice melts, the heat you put in goes into the water and the ice loses energy. Energy can also be transferred from one object to another so that one object gains energy, while the other loses energy.