There is tension about teachers' professional use of social media platforms, such as Pinterest, to access instructional resources for teaching science. Teachers' frequent use of Pinterest signals their perceived value despite concerns about the quality of the resources found there. These resources can constitute a part of teachers' network of resources, contributing in some ways and constraining in others. This study seeks to contribute to the discussion about online resources through an in-depth content analysis of 438 websites on Pinterest related to two elementary science topics: adaptation and force. Findings indicate the potential of Pinterest to expand the number and variety of examples available to teachers for both topics. However, findings also demonstrate ways resources for both topics could constrain teachers' knowledge systems: they may narrow teachers' vision of quality science instruction to engaging in activities and the resources for both topics may constrain teachers' knowledge of the science subject matter.