Throughout my 20s, I did a lot of extra hours on work projects. I'd have been doing programming anyway (it's what I love and did a lot of) and here were some ready-made problems that I could work on, that expanded my skills, and that mattered to someone.

It literally was my interest, not much different than when my kids would come to me asking me to make up a long division problem for them to work out when they were first learning multiplication and division.