How do electrons gain energy and hop levels? Does it come from shooting it with a laser or introducing heat, or some such method? By the same token, why do electrons lose energy? I assume they don't say to themselves, "I'm feeling lazy" and move down a few levels.
Also, what is the energy called that they gain and lose? Is it "quantum" energy, radiation, EM energy, or some other type of energy?
An answer that is simpler would be best. Thanks!