What exactly is resonance? I have bean reading some literature and I do not understand what precisely it is saying. But since that is very unspecific question, I will to ask some question that are puzzling me, to try to show more specific what I do not understand. 1. What exactly can resonate? Articles on resonance usually put weight on spring for the first example of resonance. But than they say glasses, springs, solids, electric circuits, ... can also resonate. What do all types of resonance have in common? 2. Co-lapsing bridge is often shown as example of harmful resonance? What exactly is resonating there? 3. What is glass's own frequency? Does every solid has "own frequency" and what "own frequency"does it mean? 4. Is some kind of oscillation always need for resonance? 5. When block is oscilating on pendulum with some frequency, the frequency is calculated by equation oscillation divided with time. If we want to calculate resonant frequency, frequency of what wold that be? 6. On Wikipedia resonance is defined as: In physics, resonance is a phenomenon that consists of a given system being driven by another vibrating system or by external forces to osccilate with greater amplitude at some preferential frequencies. What is the reason, that this happens? Thanx for answers.