Lincoln may or may not have written the Gettysburg Address on the back of an envelope.
I’ve written poems on pieces of scratch paper – anything I can get my hands on when the muse strikes.
Shane Johnson, who draws the cartoon My Life in a Cube similarly uses whatever is lying around. And this tends to reflect the theme of his cartoon (post it notes, lined paper, envelopes). You can also occasionally spot liquid paper applied artistically.
Hopefully he gets work done – whatever his work may be.