[#] This work is developed as part of the COLOS Project (COnceptual Learning Of Science), supported by Hewlett-Packard; it has been partially financed by CNR "Sistemi informatici e calcolo parallelo" and by MURST.

[*] Università degli Studi di Milano, Dipartimento Scienze Informazione, Via Comelico 39, 20135 Milano, Italy.

[1] RMG is a tool kit for authoring of visualization courseware, by C.Young of HP labs.

[2] UNIX is a trademark of Bell Laboratories, Murray Hills, NJ 07974.

[3] Objective-C is a trade name of StepStone Corporation, Sandy Hook, CO 06482.