For this demonstration, fill a microwave-safe cup or glass half full with water...
place a lightbulb, socket-end down, into the glass.
If your microwave has a rotating tray, take it out. Put the glass with the lightbulb in it as close to the center of the microwave as you can and close the door. Set the timer for 45 seconds. (Any longer is a safety hazard.)
Before removing the glass and lightbulb from the microwave, allow them to cool.
The Science Behind It?
The microwaves work the same as electrical current and excites the tungsten filaments inside the bulb.
The glass of water protects the lightbulb from the full effects of the microwaves, however, so the bulb does not explode.
Source: Steve Spangler Science