  Nanoscale confinement provides an ideal platform to rouse some exceptional
reactions which cannot happen in the open space. Intuitively, H2 and H$_2$O
cannot react. Herein, through utilizing small-sized fullerenes (C$_{24}$,
C$_{26}$, C$_{28}$, and C$_{30}$) as nanoreactors, we demonstrate that a
hyperhydrogenated water species, H$_4$O, can be easily formed using H2 and
H$_2$O under ambient conditions by ab initio molecular dynamics simulations.The
H$_4$O molecule rotates freely in the cavity of the cages and maintains its
structure during the simulations. Further theoretical analysis indicates that
H$_4$O in the fullerene possesses high stability thermodynamically and
chemically, which can be rationalized by the electron transfer between H$_4$O
and the fullerene. This work highlights the possibility of utilizing fullerene
as a nanoreactor to provide confinement constraints for unexpected chemistry.
