Sometimes the binary distributions just aren't going to cut it. While most people just need the default, you may need some package enabled, or disabled, or certain optimization flags, or you just want the control or satisfaction of having it compile. This was originally how RVM worked, it compiled everything itself, and you can still force this mode. By using the --disable-binary switch you can force RVM to download source and compile it itself.
$ rvm install 1.8.7 --disable-binary
You're not done yet. You have Linux, you have Ruby, but you need a bit more for a real "development environment," so see you in Part 3!