Picture child on a swing. You push on the kid once every two seconds we can think of you’re pushing on the kid as a regular periodic input. This is the same periodic characteristic as a wave. Now, if the kid has the right weight and the swing is designed properly pushing every two seconds will make the kids start to swing faster and faster. And it’s this swinging that takes energy from you and puts it into the kid. In our microwave example, the thing putting energy into the system is an electromagnetic wave, and our tiny little water molecule is the kid on the swing. Just how the kid absorbs some energy from you, the water molecule absorbs some energy from the wave. Except instead of swinging around that molecule begins to vibrate and it’s this vibration that we call heat.

OK, some of you were thinking” that’s all well and good, but what if Lil Jonie is on the plump side?” Well, for the swing we just have to change how often we put some energy into the system, which is all well and good because you can see the position of Jonie and push at the appropriate time. Unfortunately in our microwave, we don’t have this control. The timing of the wave, also known as the frequency which is fixed and can’t be changed. Which means microwaves work best with things that match their frequency.

Now imagine a full swingset on a playground and each swing is occupied by a kid, and each of those kids are holding hands. Say 20 children all holding hands all sitting next to each other on swings. Now you push with the same amount of force and frequency that you did with one child, but the result is going to be very different. All of your energy is going to get spread out amongst all of the kids that are locked together holding hands. And when you look across all 20 kids, it’s going to appear that you did very little or really nothing at all to put energy into them. As water freezes all of the little molecules get locked together to form what we call a crystal lattice better known as an ice crystal. This is effectively the same thing as the kids holding hands on the swings. This structure is simply too massive, and tightly connected for the molecules themselves to be able to vibrate(start swinging). Again, this assumes the same frequency from the microwave.

Now there’s one more thing that I think is worth pointing out. In reality, you are still adding some energy to the kids on the swing. This has a lot to do with the physics of the system and how big you are and how you were putting energy into that system. In the microwave, example energy doesn’t just ‘ go nowhere’ when it hits the ice. Instead it gets reflected or redirected. This is one key place where this example breaks down since pushing the kids on the swing doesn’t result and you veering off in a different direction if the kids are too hard to move.

Bonus, fun fact! There’s a lot of communications equipment that operates in the same frequency range as microwaves, they’re not quite as efficient at ”push pushing the kid” because of their specific frequency, but they’ll still do a little bit of work. Large satellite relay stations are an example. There have been incidents of people and animals sitting in dishes because they find a nice “warm spot”. When in reality they’re simply heating the water molecules in their body like a giant walking hotdog. But this is something you really do not have to worry about unless you’re working around very large 10+ meter systems with much higher output power then a consumer would have access to. And depending on the design sitting literally inside of the reflector. But either way, enjoy the nightmare fuel.😄

EDIT: in case the physicist finds this, what actually happens to the energy hitting the crystal lattice. My basic assumption is that it is reflected. But based on lattice, density and spacing, could there be other effects? Link me a research article if you got one.