Most teapots have vent holes to let steam out (and also to let air in when pouring the tea, but this is irrelevant to this question).
If the vent hole is blocked, pressure starts building up in the teapot and:
- either it pushes steam out of the nozzle, or,
- if the water level is above the nozzle entrance, it pushes water out.
(Side remark: case 2 accidentally happens to my chinese teapot when a thin film of liquid water forms over the vent, and it makes a mess if I'm not watching. I hate it!)
My question is: what is causing the pressure buildup? The pot is cooling, so if it only contained an ideal gas, pressure would go down. Is this related to the phase change rate of the hot water? What equations describe the phenomenon?