If your engine overheats and you need to add water to the radiator, you should:

• Add water immediately to cool down the engine
• Wait for the engine to cool before adding water
  Explanation:

  "Carry extra water in your car to add to your radiator if the engine overheats. Add water only after the engine has cooled. Never attempt to add water while the engine is hot!"
• Keep the engine running while adding water
• Add water slowly while the engine is still hot