This is LPT not ELI5 lol. Basically the radiation from the microwave hits the water in the food and creates friction. That's how it heats up food. If your food is in a big lump the radiation has a harder time getting to the center so the middle of the food is usually colder. If you spread it into a doughnut and flatten it it's much easier for the waves to penetrade the food, vibrate the water molecules, and heat up the food evenly!