If you mess things up and want to go back to happier times, just hold down the command key (clover leaf on Macs) and keep typing Z until you've undone as many steps as you wish. (If you want to redo what you've undone, hold down command and type R repeatedly).
And now for something slightly different ...
A box with a triangle as its first symbol is an Action Button. Double click for wonders to be unveiled. To free these treats for your exploration click once again on the Action Button (otherwise the new stuff is all selected together and will be treated as an entity).
If a table is present, rows labeled with quantities on the screen, to add to the table, move the objects in the sketch to your liking and double click on the table to add a new column of information.
Sometimes you may want to choose points in the plane which make for convenient calculations. You can turn on a grid (either rectangular or polar) in the Graph menu, but be advised that in this edition of Sketchpad, this also turns on a "Snap to Grid" feature which does just that to things you move. If it bugs you, turn it off by selecting in the same menu.
When finished with a sketch, either keep it around to refer to later or close it if you don't think you'll need it.
version of 7/12/95