  I present here a microscopic theory for the superfluidity of $^4$He (He II)
derived from experiments, and answer its essential questions. With a "momenton"
model, the superfluid is shown to feature as a "harmonic superfluid". In which
a new bonding type, the "superfluid bond", is formed. Its activation causes the
anomalous thermal excitation, the large excess of specific heat, etc. The
superfluidity mechanism is recognized being connected to a quantum confinement
effect. The theory predicts the basic properties of He II in overall agreements
with experiments. The series novel concepts evolved here reverse the current
perceptions of $^4$He, give significant impact to the understandings of other
superfluids.
