There's lots of interesting things you can do in a domestic microwave oven. Most of the following experiments are pretty safe; but atleast one of them could theoretically destroy your house, and kill everyone in it.
Also, some of these could theoretically burn out your microwave oven- however you can greatly reduce the chances of this happening by putting a small cup of water in the oven with the experiment; this absorbs most of the microwaves, and reduces the load on the magnetron- I haven't lost an oven yet anyway.
a) CD (coolness rating ***)
Place label side up on turntable and switch on for a few seconds.
- really cool sparking it looks like a special effect from a movie
- you get lots of cool fractal like burn marks on it
- CD does not play afterwards ;-)
- burning plastic smell (probably toxic- only do this in a well ventilated area)
b) Grapes (coolness rating **)
Cut a grape almost completely in half, open it and lay it down, with the two halves still connected
- you get sparking between the two halves
c) marshmallows (coolness rating **1/2)
Put a marshmallow in microwave, turn on for perhaps 4-6 seconds.
- marshmallow grows incredibly quickly
- eat marshmallow (it's very sticky and hot- be careful)
d) broken lightbulb (coolness rating **)
Put broken lightbulb in oven, switch on
- lightbulb will often light up, but with flickery, not very steady light
e) superheated water (coolness rating **)
Put a very clean, smooth cup of water in microwave heat the water for several minutes, leave to cool, repeat, with luck the water will be now superheated beyond 100C but will not be boiling. Add some sugar (very, very cautiously) to the cup
- the water should instantly, and very vigorously boil and may cover the entire inside of the oven.
- this one is actually the second most dangerous experiment, be sure to add the sugar using a long wooden spoon or something from quite a distance- superheated water has cost people their sight (one guy peered at it closely to wonder why it wasn't boiling...so don't do that).
f) melting metal (coolness rating *****)
This one is very difficult; it's also by far the most dangerous, so I won't explain it in enough detail for it to actually work. You need a >=1000 watt oven, and an 'initiator' made from graphite and iron oxide powder mixed together (alternatively pure silicon carbide as used in sandpaper or valve grinding compound is also a good choice). This forms the heating part of an extreme 'browning dish'. The browning dish is placed in contact with a small ingot of aluminum, silver or other metal (iron has been melted this way) in a crucible. The whole lot is then covered in heat proof, microwave transparent material to keep the heat in. Modifications to the microwave to stop cooling air from entering the cavity need to be made. The layout and distribution of the initiator is crucial too, otherwise the initiator will melt in which case the heating effect will have ceased.
- molten metal after 10-15 minutes (which can kill, maim, and must be kept away from all forms of water to prevent a steam explosion, which could destroy a house).
- certain forms of metal have been known to explode on heating (whether or not microwaves are used)
- metals must be totally dry before doing this, and free from voids and cavities
f) Plasmoid (coolness rating ****)
Take a wooden stick, or toothpick or something (candles are reported to work, but I was unable to achieve this, it seems to me it has to be carbon, not wax) and light it and put it under a glass bottle or jar or something. Place it in the microwave before it goes out and turn on the microwave.
- a plasmoid forms due to the carbon being excited by the microwaves, the plasmoid often fills the glass bottle and tends to rise. The plasmoid is very hot, and can destroy the bottle.
g) killing maggots (coolness rating N/A)
One time I left fish on the side for a few days in a ceramic dish with a lid on it. Trouble is the flies found it before I did, and it was absolutely crawling with about a 1/2 inch depth of maggots ewwwwwww. Anyway upon taking the lid off it riled them up and they decided to make a break for it through the ill fitting lid. I did not want them all over my kitchen. Microwave to the rescue time! I 'nuked' them.
- microwaved maggots suddenly stopped in mid-wriggle after a short while and I was able to bin the dish at my leisure.
- microwave oven smelt of maggots afterwards ;-(
h) soap (coolness rating ***)
Take a bar of soap and place in oven. Turn on.
- The bar contains fats, fats heat in ovens without any water needing to be present
- the bar expands massively, much like the marshmallow experiment above
- do not eat the soap monster