We will be upgrading the PhET servers on Tuesday, November 24 at 5:30 PM Mountain Standard Time. We expect that the maintenance will last only 60 minutes. During this time, PhET (http://phet.colorado.edu) may experience periods of unavailability. If you experience persistent issues, please e-mail the PhET Help Desk at firstname.lastname@example.org.
Thanks for your patience during our system maintenance.