The chapter illustrates three aspects for which full closure of circularity is impossible. Downcycling and dissipation are related to the loss of materials. For circulation, virgin material has to be added. Composites cannot be fully circulated by design: the better engineers fulfil their goal of a permanent connection of the components, the more invasive their separation and recycling become. Life cycle assessment is an important tool to understand sustainability aspects. Yet, simplifications are needed to reduce computing capacity, and it becomes impossible to fully describe every detail of circularity with a life cycle assessment. Science should be aware of these challenges and continue the search for the best solutions with all our strength and intellectual wealth. We should develop and use the most sustainable materials, assessing them fairly and keeping them in circulation for as long as possible.