Think microwaving a glass of water is harmless? One tiny mistake can turn it into a boiling hazard

Almost all of us do it: heat up a glass of water, some milk or the coffee left over from breakfast, perhaps out of pure habit. Every now and then, something goes wrong. There are no bubbles, it doesn't boil, everything seems normal... until the water suddenly jumps. It's not an urban legend: the explanation lies in physics, not superstition.

Manufacturers warn about it in their manuals, but the warning often goes unnoticed. On the surface, heating a cup of water seems innocuous. In reality, it is one of the few everyday gestures that can become dangerous if not done carefully.

So-called microwave overheating occurs when water is heated unevenly. In contrast to fire, which transmits heat from the base of the container, the microwave acts directly on the water molecules throughout its volume.

Under these conditions, the liquid can reach more than 210°F/100°C without boiling. No bubbles or visible steam are formed, and the water appears calm. But it is enough to introduce a teaspoon or a tea bag for the accumulated energy to be suddenly released and the water jumps violently.

A rare phenomenon, but perfectly documented.

The risk is greater with pure water, without impurities, because nothing interrupts the heating process or facilitates the formation of bubbles. On the other hand, liquids such as milk or coffee contain proteins, fats and small particles in suspension that help to disperse the heat, stabilize the temperature and reduce the risk, although they do not eliminate it completely.

The container also has an influence: new or very smooth cups facilitate overheating, while a used cup, with slight roughness or even a minimal interior mark, offers natural points where steam can be released gradually and safely.

The FDA (U.S. Food and Drug Administration) warns that heating water in the microwave without precautions can cause overheating and dangerous splashing. Although this is a rare accident, the burns can be serious and affect the hands, face or respiratory tract.

The risk comes not only from the liquid: the container itself can reach higher temperatures than expected and cause burns when touched.

It is not a matter of alarm, but of understanding how the appliance works. The microwave is safe, but with liquids, its speed can become its biggest trap.

There is no need to give up the microwave, but use it with caution. Here are some simple guidelines that greatly reduce the risks associated with overheating liquids in the microwave:

• Insert a wooden stick or a glass rod in the glass before heating: they help to release the steam little by little.
• Use medium powers and short times (20-30 s), stirring between pauses.
• Do not cover the vessel completely or fill it to the brim.
• Wait a few seconds before stirring or drinking.

As a guideline, a 250 ml glass should not be heated for more than one minute at 700-800 W, always in two or three batches.

If there is time, the traditional method remains the most stable: a saucepan or a kettle allows to control the moment of boiling and to adjust the temperature with precision, about 165-175°F/75-80°C are perfect for a green tea.