Why do you get sparks in a microwave?
Why, when I cut up apples into small pieces in a microwave close together, do they spark and flame up? If you cut a grape in half and put the two bits next door to each other in the microwave, why do you get glowing lights?
Dave: The microwaves are a form of electromagnetic wave. When they hit something they try and make an electric current flow in it. If you've got two things with a gap in between them it can produce a big current. You get a large current running across this gap or through a tiny piece of skin in between. It'll get hotter and hotter until eventually it burns out. At this point you have a big electric current trying to get between these two lumps of fruit which can jump through the air and create a spark. That will take the current and you'll get a fireworks display.