Why does boiling water in the microwave make a cup of tea go weird? When I boil water in the kettle, it makes a nice cup of tea.  Sometimes I need to use a microwave because a kettle isn't available.  I boil the water in the mug and it looks pretty normal, but when I drop in the teabag the water froths up and looks foamy.  I don't see what the chemical difference is here, so I assume it must be some physical difference.  I have noticed this with multiple types of tea and multiple microwaves, the results being consistent so it's not just a weird microwave or something like that.  
What is the reaction here and how/why does it occur?
Here is a photo of the 'fizzy' looking tea just after dunking in the teabag.  

 A: A common problem with microwave is that you lack control of the
temperature of the water. 
Second problem is that the water is heated from the top and the sides of the mug mostly. 
Result is, that the content of the mug is not really boiling hot everywhere, 
altogether (after mixing) it is well below 100 °C 
A side effect from this is, that the water is not dearated 
like it is when You boil it on a hot plate. 
So: You have some water close to 100 C at the surface 
and colder (containing air) water below. 
When You dip in the bag, all that mixes, the air bubbles out. 
A: I doubt that it's superheating, as I understand superheating is a rather violent phenomenon. The most likely explanation is dissolved gases; gas solubility in water decreases with temperature. The kettle boiling process is very turbulent and so can release all the dissolved gases, resulting in pure hot water. The microwaved water is heated in a very gentle way, and so the gases do not yet have a chance to escape from the surface... not until they can start nucleating on your tea bag.
A: http://en.wikipedia.org/wiki/Superheating
I think you are superheating the water and you provide nucleation sites (by means of the tea bag) so it starts to boil.  
A: Another problem here could be the temperature of the water. If you pour boiling water into a cold mug, then the temperature of the water will drop due to heat transfer to the mug. Also as you are pouring some of the water turns to steam. 
If you boil the water in the mug, then the water in the mug is at the boiling point. 
Hotter seeping temperatures tend to bring out bitter flavors in the tea. 
A: I noticed the same thing, and assumed it was because of superheating damaging the tea, as you can't do much to H2O chemically, except change it to hydrogen peroxide, which has a nutty taste. So I tried adding some cold water to bring it down to 100 deg., but without success. Superheating will cause it to boil when something rough is inserted, but it fizzes like soft drink (soda in American). A man from the Lipton company was on the tele. some years ago saying about the importance of the oxygen in the water to the taste of the tea (I didn't think there would be any dissolved gasses left in boiled water, but apparently, there is).
I think what happens is that the microwaves, or the current induced by them separates the gasses out as micro bubbles, that are in suspension, and cause the fizzing, like the CO2 in soft drink. I found, if you are reheating cold tea, put any additives in, like sugar and/or powdered milk in first and stir well so there is no particulate matter to seed bubble formation. Heat it slightly extra and leave it for a few minutes, so the micro bubbles re-dissolve, and there will be little effect on the taste.
