Step 17. Nested Loops
The first World draws a bunch of circles every step, with a moving offset.
The second World uses a loop inside a loop to draw a grid of circles.
Instead of a full grid, it is a grid from x=0 out to the x-value of the mouse position.
As with all the Worlds, you can edit it to add whatever code you see in the video,
and whatever new code you can think of.