I heat food in my microwave pretty much every day of the week. One of my friends said microwaves can make food radioactive. Surely that's not true?"
There are many different types of radiation that have different levels of energy. X-rays have lots of energy and potentially can ‘break’ DNA. This is known as ionising radiation and, especially with accumulated exposure over time, can contribute to cancer risk.
Microwaves, radio waves, and the light that we can see, are all examples of non-ionising radiation. The only non-ionising radiation which causes cancer is ultraviolet (UV) light, which is why people are advised to protect themselves from excessive sun exposure when UV levels are high. Microwaves are not known to cause cancer.
Microwave ovens use microwave radiation to heat food, but this does not mean that they make food radioactive. Microwaves heat food by causing water molecules to vibrate and, as a result, food is heated. This is different from absorption of heat energy by food which is the basis of conventional cooking.