Here's why:

* Ammonia (NH₃): Ammonia has a nitrogen atom with 3 single bonds to hydrogen atoms and one lone pair of electrons.

* Ammonia ion (NH₄⁺): When ammonia gains a hydrogen ion (H⁺), the lone pair on nitrogen forms a fourth bond with the hydrogen ion.

This results in a positively charged ammonium ion (NH₄⁺) with a tetrahedral geometry.