Why does steam in a teapot move to the largest opening? This morning I was making tea, and I took my teapot off of the stove and opened the spigot (where the front / whistle clasps onto) to let it cool. When I opened the spigot, steam started coming out as expected. However, when I opened the top of the teapot, a significantly larger opening, the steam began to pour out of the top and none of it (visibly) out of the spigot.
I assumed this is related to thermodynamics, but can’t find an exact explanation. Is there some law that dictates that gasses will always choose the path of least resistance? I know that’s true of electrons, but haven’t found something for gasses.
 A: It's just a question of least resistance in this case- it takes a tiny bit less work to push steam through that big opening than it did to push it out the spout.
A: It’s possible that your teakettle’s second opening makes it act as a chimney. The heat transfer might be more efficient if high-density cold air enters the small opening to replace the low-density hot air exiting the large opening.
There are certainly kettle geometries where you would get a chimney effect. For example, if you boiled water in your fireplace, opening the flue would make nearly all of the steam go up that actual chimney, to be replaced by air from your living room. (Pro tip: do not operate your fireplace with the flue closed.) There are also geometries where you would expect no chimney effect: for instance, if the two openings were symmetrical. Whether this effect holds in an “ordinary” teakettle seems like a computational question; there are lots of different versions of “ordinary.”
I’ve just tested this by boiling a kettle, holding a lit match at the whistle, then removing the lid with my other hand. With the kettle lid on, of course steam is blowing furiously out of the whistle and will blow the match out. With the kettle lid removed, it seems to me that the flame is pulled towards the hole, which supports the airflow-in hypothesis. This isn’t slam-dunk evidence — and now I’ve burned my hand trying to take a photo, so you’re either going to have to trust me or to get your own matches — but it’s promising.
