Answer
The high pressure inside the sealed radiator raises the boiling point for water. The water doesn’t boil, even though its temperature is greater than $100^{\circ}C$. This is the same principle behind a pressure cooker's operation.
When the radiator cap is removed, the pressure inside is released and the high-temperature water immediately begins to boil, resulting in a small explosion. Give your car's radiator a chance to cool down before slowly removing the cap. If you must remove it while the radiator is hot, make sure to use thick gloves, and stay out of the way of the outlet.
Work Step by Step
This is discussed on page 326.