Alright, reading a paper, and I witness this

We like math

We got that in common

My vote for you would be the countable vs uncountable infinity thing. You could even talk about the unsolved continuum hypothesis. There's not a "whole lot" of math involved (it's fairly standard to mention in any discrete math course) and there's a wide range of involved topics you can mention (cardinality of sets (some infinities bigger than others), cardinal numbers vs ordinal numbers, completeness in real analysis (this is a very important and fundamental consequence of the properties of real numbers) etc).

I should add, basically, the integers and rational numbers are countable but the real numbers are not. What this means: the interval [0,1] (of real numbers) is the same size as the whole set of real numbers and the natural numbers, integers, and rationals are the same size.

Ay, thanks