how do we know that there are just two types of charges

I'm planning to start the semester off by challenging the students with the question "how do we know that there are only two types of charges". May be useful to emphasize the nature of science, but also may go over with a thud, depending on whether students are already convinced that there are only two types of charges.

Note that it took from ~600 BC to 1800 AD, almost 2500 years, for humankind to reach the understanding that there are just two types of charges. And I plan to do this in ~30min!