What is another metaphor (aside from the fact that a piano can only produce certain notes) that could help someone understand enrgy quantization at the atomic level? Explain your answer. The text it is referring to is: Because protons and electrons in an atom carry opposite charges, they are attracted to each other and thus maintain the boundaries of the atom as a singular closed system. This means that an atom’s energy is quantized, or fixed to a certain amount. Just like a piano can only produce a certain set of notes that correspond to its keys, an atom of a certain element can only contain a certain fixed amount of energy.