  We propose modifying large water \v{C}erenkov detectors by the addition of
0.2% gadolinium trichloride, which is highly soluble, newly inexpensive, and
transparent in solution. Since Gd has an enormous cross section for radiative
neutron capture, with $\sum E_\gamma = 8$ MeV, this would make neutrons visible
for the first time in such detectors, allowing antineutrino tagging by the
coincidence detection reaction $\bar{\nu}_e + p \to e^+ + n$ (similarly for
$\bar{\nu}_\mu$). Taking Super-Kamiokande as a working example, dramatic
consequences for reactor neutrino measurements, first observation of the
diffuse supernova neutrino background, Galactic supernova detection, and other
topics are discussed.
