All sorts of things.

The number of labelled trees with n vertices is n^(n-2), so there ought to be a simple and natural correspondence between labelled tress and "words" of (n-2) letters chosen from an alphabet of n. What is it?

Newtonian physics has a notion of gravitational flux which seems to be missing from general relativity. How and under what conditions can I define a relativistic gravitational flux?

What use are quaternions?

What use is hyperbolic geometry?

Linear systems of the form A™ C A x = b occur in many contexts. Yet there doesn't seem to be a really good way to express x as a function of the diagonal matrix C when A and b are fixed.