Strength identification in conjugate acid-base pairing
$\ce{H2O}$ is a weak acid.  Abstract a proton to give $\ce{HO^{-}}$, a strong base.  Abstract a proton from that to give $\ce{O^{2-}}$, a very strong base.  See a species as a whole, up and down for a property.  

Try it with ammonia.  Feel free to wonder why $\ce{Li3N}$ is red.  Look what a very strong base can do,

$\ce{Li3N + 2 H2 → LiNH2 + 2 LiH}$ (reversible at 270°C)