  We propose a minimal model describing magnetic behavior of Fe-based
superconductors. The key ingredient of the model is a dynamical mixing of
quasi-degenerate spin states of Fe2+ ion by intersite electron hoppings,
resulting in an effective local spin Seff. The moments Seff tend to form
singlet pairs, and may condense into a spin nematic phase due to the emergent
biquadratic exchange couplings. The long-range ordered part m of Seff varies
widely, 0<=m<=Seff, but magnon spectra are universal and scale with Seff,
resolving the puzzle of large but fluctuating Fe-moments. Unusual temperature
dependences of a local moment and spin susceptibility are also explained.
