Here's why:

* Conjugate acid-base pairs: A conjugate acid-base pair is a set of two molecules or ions that differ by a single proton (H⁺).

* Acid-base reactions: When an acid (like H₂O) donates a proton, it forms its conjugate base (OH⁻). The reverse is also true: When a base (OH⁻) accepts a proton, it forms its conjugate acid (H₂O).

So, in the reaction:

H₂O ⇌ H⁺ + OH⁻

* H₂O acts as an acid, donating a proton to form OH⁻ (its conjugate base).

* OH⁻ acts as a base, accepting a proton to form H₂O (its conjugate acid).