Back in 1993, after learning GW-BASIC in high school, I got QBasic and 'hacked' nibbles.bas that was shipped with it. I added speed increase/decrease at my will, and similar stuff for fun. :-)

I printed all source-code and for days and hours I was dissecting it.

Later on, I learned Pascal, C, some Mathematica and Maple, LaTeX, GnuPlot, HTML+CSS, JavaScript, Perl, PHP, some C++.