Three's the charm, they say. Unfortunately I had to go through that cycle 6 times before getting here. Meaning this is exactly the 18th puzzle I ever constructed, but my first ever for the New York Times. So, good to be here.
As a first timer, perhaps I should say I work for the UN, I've lived overseas for many years in many different places (currently Bangkok), and my background is in engineering, philosophy and international affairs. Somehow those three fit together well enough. These days I mostly focus on environmental issues.
So contemplating groups of threes, I think I was sitting in a meeting one day having trouble concentrating. So my mind drifted toward groups of threes, as it does. There are so many! From the Wise Men to the Blind Mice to the Little Pigs, from Three Dog Night to Three Days Grace, from the Stooges to the Musketeers to the Holy Trinity. And in the inanimate world the list continues: there's earth, wind and fire; the sun, moon and stars; rock, paper, scissors; and of course … liquid, solid, gas.
Now most of these potential themes have been worked over pretty well already. But the cruciverb database told me that STATESOFMATTER was as yet unused. As well, this nice 14-letter word is exactly the same length as LIQUIDSOLIDGAS. So I was off and running. Choosing H2O as the matter in question, I looked for phrases that included ice, water and steam that hadn't been overused already.
ICESKATINGRINKS had never seen used, runs to 15 letters and could go smack dab in the middle of the puzzle. So putting that in place and with the other themers as my guide, it turns out that WATERTANKS and STEAMBOATS are both 10 letters and fit perfectly and symmetrically at 11D and 26D, locking all the theme words together quite nicely I thought. So there you have it.
One last point for the purist in us all: yes, there are more than three states of matter. If this puzzle had a title, it would have been "Forget Plasma." According to Wikipedia, I guess we should ignore "Bose–Einstein condensates, neutron-degenerate matter and quark-gluon plasma" as well.