Colin Sullender originally shared to Science GIFs:
Generating plasma inside a microwave
Plasma is one of the four fundamental states of matter (others are solid, liquid, and gas) and the most abundant form of matter in the Universe. It is comprised of ionized atoms with either a positive or negative charge and free electrons.
As the match burns, electrons are torn from their atoms when chemical bonds break within molecules. Normally those electrons would immediately be re-captured to form the byproducts of combustion. However, the energy provided by the microwave radiation causes this cloud of intermediate ions and free electrons to persist in a semi-stable state. The intense light is a result of these high-energy particles losing energy before being re-energized by the microwave. When the microwave is turned off, the plasma will immediately dissipate since it no longer has the requisite energy to exist.
WARNING: Doing this could damage your microwave!
Source: Microwave Plasma : AWESOME experiment, Stupideaproductions Microwave plasma