How it Works

• Metals reflect electromagnetic waves used to agitate water molecules. Thin scraps of metal, such as fork prongs and tin foil, behave like an antennae inside a microwave oven. Microwaves bounce off the metal, rather than penetrate it, causing sparks.
  
  

Objects that Will Spark

• Pointed edges of metal are the key to whether sparks occur inside your microwave. While fork tines deflect the microwaves and cause an arc of energy that connects the metal to the walls of the microwave, other metal items, such as a staple, can overheat and cause a fire, but wont cause sparks.
  
  

Avoiding Sparks

• While you should completely avoid putting metal in a microwave at all, the Health Physics Society reports that sparks and overheating from metal objects can be avoided if the metal is inside the food or in water.