Watch the Day 15 online guitar lesson by Ravi from Learn Guitar in 21 Days
This is a big day, because minor chords really blow the song list possibilities wide open. By mixing majors and minors, so many more songs are now possible. As you'll see, this is a very common songwriting technique.
So, explore the sounds of majors with minors, and you might soon be writing some very hum-able tunes yourself! Still using only three-chord songs, the minor chords make many more available to you.
Tackle songs that you haven't tried before and make sure that they each include at least one minor chord.