H₂O is the chemical sign for water. It shows what water is made of: two atoms of hydrogen and one atom of oxygen. The little 2 (written low, as a subscript) belongs to H and means “two of those.” You read it “H-two-O.” People write H₂O on the board, on some bottles, and in jokes, as a clever way to say water. Though actually, the 2 is not the number twenty — typing H20 is a common slip. Chemists have signs like this for many substances; H₂O is the one almost every student learns first. In daily life you still just say water; H₂O is the science label.

水的化學式