Left and right arrow keys. Scrolling can seem difficult at first so this is as simple an example as possible. The tree has a real world position. As we 'move' Scratch cat our window on the real world changes which is recorded by scrollx. So scrollx is just an adjustment to the real world position of things to indicate where in our window it would appear. So the tree doesn't move in the real world but our view of the world has changed.
Lesson 2 is at https://scratch.mit.edu/projects/398086031/
