The reason that rain water makes your hair curly is the rain water contains hydrogen and the hydrogen then bonds with the Keratin in your hair.
The Keratin is a natural protein that occurs in your hair and when that hydrogen bonds with the Keratin Protein it causes your hair to become curly.
The rain water can also cause your hair to become frizzy because the hair shaft swells as a result of the rain waters hydrogen.
That's why water is called H20 as the H stands for Hydrogen.
H20 means Hydrogen Atom, Oxygen.
H20 is the scientific name for water which we just know simply as water.
Water is very important to everyone on earth as we could not live without water.