Why does foil cause sparks in a microwave?

[GunUltimateID]

I put a hamburger(wrapped by foil) into microwave and pressed start. I noticed an electric spark near the foil's surface. How can we explain this incident?
ps. sorry for my english

 

[torquil]

http://en.wikipedia.org/wiki/Microwave_oven#Metal_objects

 

[GunUltimateID]

thank you very much