As the microwaves used to heat food bounce around inside the oven, they create patches of high and low intensity – and thus uneven levels of heating. The problem with soup is that it tends to be relatively gloopy (or ‘viscous’), slowing the diffusion of heat within it. As a result, some parts of the soup start to form expanding bubbles of hot vapour which can burst through the surface, making a mess. Stirring the soup halfway through often solves the problem.
Robert is a science writer and visiting professor of science at Aston University.