"freely used and/or redistributed by third parties (e.g. students, educators, school districts, museums, publishers, vendors, etc.). Non-commercial or commercial use is allowed. All uses require attribution of the work as describe above."
"In addition, the source code for all PhET simulations is available for use and/or modification. For directions about how to access the source code, go here. Anyone can have access to the source code and make modifications. The source code for any changes someone makes to the software must, in turn, be made publicly available by the party that makes the changes."
"freely used and/or redistributed by third parties (e.g. students, educators, school districts, museums, publishers, vendors, etc.). Non-commercial or commercial use is allowed. All uses require attribution of the work."