Here's how it happens:

* Water is a polar molecule: This means that the oxygen atom has a slightly negative charge, while the hydrogen atoms have slightly positive charges.

* Self-ionization: Water molecules can interact with each other. Sometimes, a hydrogen ion (H+) is transferred from one water molecule to another.

* Formation of ions: The water molecule that loses a hydrogen ion becomes a hydroxide ion (OH-), while the water molecule that gains the hydrogen ion becomes a hydronium ion (H3O+).

Here's a simplified representation:

H2O + H2O ⇌ H3O+ + OH-

This reaction is reversible, meaning it can go in both directions. However, in pure water, the concentration of both hydroxide ions and hydronium ions is very small, resulting in a neutral pH.

Key takeaway: Water's ability to ionize is crucial for many chemical reactions, and the formation of hydroxide ions is a key factor in determining the pH of a solution.