Tom is a sometime crossword constructor. I suggested the following theme to him.
Horizontal theme answers: EVOLUTION, SELECTION, MUTATION, FITNESS, DESCENT, WALLACE, DARWIN, for example.
Intersected by vertical theme answers: PHYSICS, CHEMISTRY, GEOLOGY, ASTRONOMY, CHEMISTRY, GENETICS, for example.
A beautiful metaphor for what science is and how it works. Science is not a smorgasbord of ideas from which one can pick and choose. Any scientific idea takes its significance and resilience from its place in a mutually reinforcing web of ideas.
Tom wittily suggested instead a crossword that has only a single theme answer: INTELLIGENT DESIGN.