Udemy, which allows users to create their own online courses, has raised $113 million with a model that charges users a fee and pays teachers based on popularity—some more than others. Take Alun Hill, who used his earnings to move to a fortress in Malta after he recorded a series of popular classes (including one on how to make money on Udemy), which he says still brings in $1,000 a day. Former high school math teacher Rob Percival, the company’s top instructor, has made more than $2 million since 2014. And Percival’s friend and tennis partner, Ben Tristem, says he earns in the high six figures teaching game development.

Not everyone gets to Malta, though: Average Udemy teacher take-home pay is $8,000.

A version of this article appears in the November 1, 2015 issue of Fortune with the headline “Meet the guys making bank by teaching online.”