Make a search on the Web for the terms Ivan Sutherland + Sketchpad. YouTube, archive.org, or some other site should have the video. Hopefully, YouTube will have the Ivan Sutherland : Sketchpad Demo (1/2) and Ivan Sutherland : Sketchpad Demo (2/2), but any of them will do. Sketchpad Demo (2/2) is best. Watch it if you can find it. If you watch Ivan Sutherland : Sketchpad Demo (1/2) you can skip the first 3:33 part of the video unless you enjoy 1960s technical jargon.
Watch it now and enjoy it.
We saw the grandfather of all computer animation programs. Similar to early ink animations, it was all done with lines.