Changing the default font on the fly

[Eddie <penguinismo@almostconnecticut.net>]

The default font in my Emacs 23.3 (various Linux installations) is
DejaVu Sans Mono Book 11. That's perfect for the calendar and my phone
book, etc. if I want to do some longer writing in a more readable
font, I can use the menu and go Options | Set Default Font | nibble
through the various choices.

But I'd rather not use the menu. I would like to get a hunk of code
that is the functional equivalent to Options | Set Default Font | Droid
Sans Regular 12. 

I don't know how to accomplish this on my own (I'm primarily a writer
and editor, and I've never demonstrated any talent for programming),
nor have I found any examples online that I could modify for my own
uses. 

And from there I would like to assign that hunk of code to a keystroke
series. I've assigned some macros to keystrokes, so I think I can do
that part.

Any suggestions would be gratefully appreciated.