Cognitive tools, defined here as instruments that support or perform cognitive processes for learners in order to support learning, can bridge the difference between open learning environments, like discovery learning environments and traditional supportive instructional environments. This article discusses a definition of the concept of cognitive tool and its use in learning. Two examples of cognitive tools for discovery environments are presented, and it is made clear how these tools can serve as hooks for anchoring intelligent instruction. Finally design issues for integrating cognitive tools in a discovery environments are discussed. (http://aied.inf.ed.ac.uk/members99/archive/vol_10/joolingen/full.html)