Why cant you put foil in a microwave?

The foil will spark and possibly set the microwave on fire. NINJAWOLF99's answer:

When I put maynase packets in the microwave to heat them up, after only two seconds, a large, blu flash could be seen on the inside. I was VERY lucky the thing didn't set on fire. I took them out. 1) They did not melt 2) They still felt cold.

Answer: No.