We will be upgrading the PhET servers on Friday, May 24 at 6:00 PM Mountain Daylight Time. We expect that the maintenance will last only 30 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.