Ah, thank you! It wasn't clear to me that dragging was part of the touch UI at all. I thought it was "tap square, tap letter to put in square." And then I could

Ah, thank you! It wasn't clear to me that dragging was part of the touch UI at all. I thought it was "tap square, tap letter to put in square." And then I couldn't find anything to tap to remove a letter from a square.