Step 1: Identify calcium oxide by its common name.
Calcium oxide, $\text{CaO}$, is widely known as quicklime, and it is famous for reacting strongly and quickly whenever it meets water.
Step 2: Picture what happens when water is poured on it.
The reaction is so vigorous that it hisses and gives off a large amount of heat, this is why quicklime must be handled carefully around moisture.
Step 3: Write the reaction. \[ \text{CaO} + \text{H}_2\text{O} \to \text{Ca(OH)}_2 + \text{heat} \] The oxide combines directly with water in a simple combination reaction.
Step 4: Name the product.
The compound formed, calcium hydroxide, is commonly called slaked lime and is the same substance used later for whitewashing walls. \[ \boxed{\text{Calcium hydroxide}} \]