Re: Why is it that aluminum foil does not get hot when placed in the oven?

A frequency of 2450MHz, which equals a wavelength of approx. 12cm, radio 
waves is emitted from the magnetron in the microwave oven. The molecules 
within the food ¨C especially the polar water molecules, but also 
aminoacids, lipids and proteins adsorbed microwaves and are forced to 
vibrate at 2,450,000,000 times per second with the rapidly changing 
alternating electrical field. This movement or vibration generates heat 
for cooking due to intermolecular friction. Thus, the food is heated from 
the inside outwards leaving the dishes and the oven itself cold, because 
they are not directly heated by microwaves. 
Ceramic and glass containers allow microwaves to penetrate through them 
without being heated making them as ideal containers for microwave 
cooking. However, Microwaves are reflected back by metal and as such you 
must be very careful not to use metal containers in microwave cooking. 
Because fat and bone is more easily cooked than other parts, you can cover 
the fat and bone parts using aluminum foil and remove it after a few 
minutes when cooking meat with much fat and bone that are not evenly 
distributed. 

You can learn more about the principle of the microwave at http://www.wffns.org/journal/artikel/mikro_en.html http://hzdf.pages.com.cn/cai/zhao/zhao16.html