Erupting, or violent boiling over, can occur when a liquid (primarily water) heated in a microwave oven becomes hotter than its conventional 212 degree Fahrenheit boiling temperature (super-heated).  A superheated cup of water could theoretically erupt violently when it is removed from a microwave oven.  Superheating usually occurs when microwaving water in a very clean vessel – usually just taken from a dishwasher – or when microwaving room temperature water.  Several precautions can be taken to lessen the potential for a problem:

• Use a vessel with sloping walls, such as a measuring cup.
• Leave a microwavable spoon in the vessel while heating.
• Stir occasionally while heating.