This reminds me of my undergrad “discrete math” lecture notes, where my professor wrote the following in the unit on sets:

> This brings us to a fun problem for you to think about in your spare time: are there any infinite sets that have cardinality strictly between [the cardinality of the natural numbers] and [the cardinality of the real numbers]?

This is of course the famously undecidable continuum hypothesis (https://en.wikipedia.org/wiki/Continuum_hypothesis).