  Ab initio electronic structure calculations show that the recently identified
quasi-two-dimensional electride Y$_2$C is a weak itinerant ferromagnet or at
least close to a ferromagnetic instability. The ferromagnetism is induced by
the electride electrons, which are loosely bound around interstitial sites and
overlap with each other to form two-dimensional interlayer conduction bands.
The semimetallicity and two-dimensionality of the band structure are the key to
understanding this ferromagnetic instability.
