Why does tea spill ?

We all know why does milk spill when overheated...Apparently the layer of cream formed on the milk surface is to blame.
Well I do not fully agree with it but then it is the most plausible reason so lets accept that as an axiom.
Now, do a small experiment. Heat water in a small teapot. make sure teapot is at least 50% filled with water (not that this is a mandatory condition for the result but it will make result sufficiently clear coz you will have to take the pain of cleaning up the stove after this :-D). When water starts boiling add tea powder. Of course, be reasonable while doing this. Don't empty entire box but don't be frugal either (best option do this when u r making ur evening tea). Don't bother to reduce the flame. You will see that tea does spill just like milk. Now keep stirring the tea while its boiling. No matter how fast you stir it still tries everything to spill out of teapot.
Any guesses why this happens. If you know the answer, please be kind enough to share it with me. I am dying to find it out.

If you think this is a totally pointless thing to do....then.... may be it is. But don't forget a similar observation had resulted in the invention of first steam engine.