You can ?quantize? your whistle by whistling down a long tube, such as the tube from a roll of wrapping paper. First, without the tube, whistle from a high pitch to a low pitch. Do it loudly and in a single breath. (If you can?t whistle, find some one who can.) Next, try the same thing while holding the tube to your lips. Ah ha! Note that some frequencies simply cannot be whistled, no matter how hard you try. These frequencies are forbidden because their wavelengths are not a multiple of the length of the tube.Try experimenting with tubes of different lengths. To hear yourself more clearly, use a flexible plastic tube, and twist the outer end toward your ear.When your whistle is confined to the tube, the consequence is a quantization of its frequencies. When an electron wave is confined to an atom, the consequence is a quantization of the electron?s energy.