Microwaves are electric fields just like the electric fields that you make by scuffing your socks on a carpet, or the ones that make electrical currents flow through the appliances you plug into the wall. The only difference between these electric fields and microwave electric fields is that the microwave fields jiggle back and forth very quickly - about a billion times a second. This jiggling is at exactly a rate at which water molecules like to vibrate. So when a water molecule feels this electric field jiggling back and forth at this rate, it vibrates very strongly. The friction between the water molecule and whatever is around produces heat. So a microwave oven works by heating the water molecules inside the food, which then heat the rest of the food. This means that very dry things (like bread) do not cook well in microwaves, while things with alot of water in them (like soup) get quite hot.