Three main issues can be listed from the literature about computer-supported cognitive tools. Such tools are frequently evoked for improving learning, but seldom for assisting teaching. They are in most cases an application of an already-designed tool (e.g., concept maps) and models for designing new ones are lacking. Moreover, the underlying model of the activity of teaching they use is often function-based (i.e., a cognitive tool for teaching a given subject matter) than capacity-based (i.e., a cognitive tool for fostering a given teacher's capacity). We present here a model for designing computer-supported cognitive tools that aims at addressing these three issues. This model uses Popper's theory of knowledge, which is suitable for the design of knowledge-oriented tools. Three levels are successively considered in the design of such tools: the material world 1, the artifactual world 3, and the cognitive world 2. University students in instructional technology were given this framework and had to follow it for designing their own cognitive tool for teaching. An analysis of these tools shows that our model allows specifying a wide range of cognitive tools, without constraining the creativity of their designers.