Click and drag in the canvas window. The faster you drag, the bigger the circles are. Best viewed in Chrome or Safari.
1) Enable the checkbox - checking it will cause all of the circles to be drawn at 50% alpha.
2) Create an additional variation of this speed circle theme, or on a similar theme such as symmetry. Tell us what the variation is so that we can read it right here: By dragging the slider you are able to control the symmetry of the circles and the size of the circles depends on how fast you drag them around the screen
Check out the Khan Academy examples (written in processing) here: Speed Circles & Symmetry
Here's our original Speed Circles JS, and in-class variation example: Speed Circles JS 2.