This morning on Public Radio's Performance Today, I heard about Mozart going back and forth with [another composer] with colorful sketches, markings, and quips within his original manuscripts like a couple 4th graders would.  I was inspired and intrigued to find out more and see for myself.

Turns out you can peruse a digital archive of Mozart's and LOADS of other artists' original manuscripts and artwork on-line!  By default, you are led to a crafty Shockwave viewer that lets you zoom, pan, and flip pages, but there is also an accessible version that might be easier to digest (and faster to dive into!).

Note, in the short time I spent there, I never did find the pages and colorful text referenced on PT, but I welcome the incidental discovery all the same.  If you know where I can see the originals described by Fred Child, please comment below!