Why Do Microwaves Cause Hotspots?

Like water waves, microwaves have peaks and valleys. The heating only happens at the peaks, not in the valleys. Microwave ovens have turntables that slowly rotate the food to move the food from wave valley to wave peak and so achieve more even heating. As you know, even with food that is being cooked for several minutes, this still can leave spots that are colder and spots that are really hot. 

Because of their small volume, the milk/formula in baby bottles can be heated very quickly. The faster the heating, the bigger the risk of hotspots, and the greater the needs for a fast turntable. A conventional turntable in a microwave spins around 3 times a minute (3 rpm), while Whirly Warmer’s turntable spins more than 100 times faster at around 400 rpm. This creates a gentle whirl of the milk or formula in the baby bottle, constantly mixing it and hence dissipating hotspots before they can even form, for completely even heating.

By the way, any other warming process, like in conventional baby bottle warmers, also causes hotspots. Typically the milk or formula near the outside of the bottle - close to the warm water or steam used in conventional warmers - gets heated first, while the milk/formula closer to the center of the bottle is cooler. A frying pan also causes hotspots, which is why you have to regularly turn over the steak you cook, to even out the heating.