You've probably noticed that your microwave heats unevenly. It's a frustrating little thing that makes a difference in your meal. There's a simple way to find out where the hot spots are exactly. Once you know where the highest concentration is, you can position your dish there for more even cooking.
Finding the hot spots is a lot of fun, even if you don't care how unevenly your food cooks. First, place a plate covered in marshmallows into your microwave. To make this extra fun, trade out the regular marshmallows and put Peeps on the plate instead.
Set the microwave for 50 seconds on high and watch through the microwave window. The marshmallows that puff up first are in a hot spot.
The science behind it
To heat food, electromagnetic waves are transmitted from a vacuum tube called a magnetron in the top of the appliance. These microwaves make the water molecules in the food vibrate, and the friction from the vibration creates heat.
The microwaves aren't sent out in a uniform pattern that envelopes the entire dish, though. There are high and low electric field intensity spots of microwaves. The low electric field intensity will causes cooler spots than the higher intensity spots. The hot spots are in the higher intensity areas.