Have you ever wondered how microwaves work? Do you find it easy to explain the concept to a 12 year old kid? Well, Here is one way to explain it so that everyone will understand.
Just try to imagine the following analogy. Let's say that we have a room full with people that are attending a party. We switch the radio on and try to find a radio station (by changing the frequency) that has music all these people like. On one radio station (one specific frequency where this radio station transmits its program) the music that is being played is classical music. Of course nothing happens as this kind of music is not to everyone's liking. We then switch to another radio station (another frequency) and what we hear from the speakers is a discussion about politics. Again nothing happens as politics is not the best background sound for a party.
We switch radio stations (by changing the frequency of the radio) until we find a station that plays disco. What happens then with the people at the party? Well, we have found the best music that most people enjoy (this is up for debate but try to follow my reasoning here). So people start to dance and bump onto each other under the influence of disco music. As they dance they produce heat. That emitted heat eventually increases the room temperature.
This is exactly how microwave ovens work. Just replace the word "room" with "soup" and the word "people" with "water molecules". At a specific frequency of the electromagnetic spectrum we have microwave radiation. That part of the spectrum can be considered as the radio station that plays disco music and the microwaves themselves can be considered the music of a specific frequency. As the water molecules love the "music" of that frequency they start to vibrate more and more. Think about these vibrations as the dance moves that the people in our example perform when they hear disco music. As the water molecules vibrate the collide with each other thus producing heat which in turn heats up the soup.
I hope the above explanation makes sense. If not then have a look at the clip below.
And if you want to watch a clip that helps you see the microwaves inside an oven have a look at the following clip.