Which should I teach first, chords or melodies?

Chords are comprised of notes, so I believe that simple melodies and "mini scales" be taught first.  A good example would be "Happy Birthday" played entirely on the first string.  A scale can be as small as three notes (e,f,g) on the first string.  Once the fingers are moving around I present the C, F, and G.  The F is abbreviated-f,a,c,e and so is the G-g,b,d,g,b,e.