Just space. I draw a LOT, so don't feel bad if yours doesn't turn out so good, though I doubt it will! Paint is a surprisingly tricky thing, and you must take the time to learn its strange ways ;) NOTE:This is rather outdated. I don't draw cats like this anymore. I'll be making another one for Paint, and GIMP, and Photoshop Elements 7, and maybe I'll try vector for you. But let me say one thing about vector: don't ever expect it out of me. I just don't like it. Not that I don't like vector art or respect those who do perfer it, I just don't like to draw in vector myself. I always prefer raster.
I used Paint (obviously) I doesn't end with "Scratch on"... space once more for a sneak peek.
