You can thank Reddit for this revelation: you've been microwaving leftovers wrong your entire life.
Ever heat up, say, a plate of spaghetti in the microwave? You nuke it for 1-2 minutes, take it out, notice the noodles in the middle are still cold, so you mix it up, toss it in for 30 more seconds, then burn your mouth when you finally take a bite?
Turns out microwaves can't effectively penetrate piles of food like pasta or leftover Chinese, which is why the heat up but the center doesn't. Next time, arrange your noodles in the shape of a doughnut (a circle of pasta with a hole in the middle). That way, it'll heat more evenly.
This trick also helps with foods like drumsticks. Arrange them in a circle so the meaty part is near the edge of the plate and the bone is facing in toward the middle.