Marianne Hu asked about the advisability of microwaving milk and
wondered if there could be a problem with residual radioactivity.
Microwave ovens do not produce or use radioactivity. The "radio" in
radioactivity is pretty much meaningless using the 1990's definition of
radio.
A microwave oven is a radio transmitter in a box. Radio waves are
produced by a large vacuum tube (the magnetron). The food is
placed in a cavity on the output of the magnetron. The food is heated
because fat and water molecules in it are being "jiggled" several
billion times a second the the radio waves. This is not and has nothing
to do with radioactivity.
Radio waves, infrared, visible light and x-rays are part of the
electromagnetic spectrum. Radio waves have the lowest frequencies
followed by infrared (from stove burners among other things) have higher
frequencies followed by visible light, ultraviolet and x-rays.