Over the course of my doctoral work in granular materials, geophysical fluid dynamics, and continuum mechanics, I kept a running document of interesting problems — questions I encountered in papers, lectures, and my own research, along with questions I found worth asking. This is that collection.
It is not a textbook or a problem set. It is more like a notebook of curiosities: things I could not immediately answer, things I wanted to think more carefully about, and occasionally, things I eventually did answer. It is organized loosely by topic and grows with time.