Heating metals in a microwave leads to different results because metals are electrically conductive. Heating may or may not occur, depending on the form of the metal. Bulk metals (such as aluminum foil and solid steel) at room temperature reflect microwaves due to low penetration depth, setting up a high voltage between the metal and the magnetron. When this voltage surpasses a threshold, it discharges as a visible spark. For that reason, it is advised not to put metal articles into microwave ovens.

Yup, lesson learnt. Praise the Lord, I'm not taking Home Econs next year!