Each of the three circles can be On, with its interior painted, or Off, with the exterior painted, or neutral adding a contour without painted regions. The circles can be dragged and resized.
(This applet requires Java 2: jre 1.3.1, at least. You can try to log into www.java.com for an automatic download of the latest available jre, or, for a manual download, try www.sun.com. Pick the latest Standard Edition - J2SE.)