Vygotsky's Zone of Proximal Development (ZPD) provides an important understanding of learning, but its implications for teachers are often unclear or limited and could be further explored. We use conceptual analysis to sharpen the ZPD as a teaching tool, illustrated with examples from teaching critical thinking in zoology. Our conclusions are the following: teachers should assign tasks that students cannot do on their own, but which they can do with assistance; they should provide just enough assistance so that students learn to complete the tasks independently and, finally, teachers can increase learning gains by providing learning environments that enable students to do harder tasks than would otherwise be possible and by assigning the hardest tasks students can do with assistance. This analysis provides a sharp and useful tool for supporting learning across all curriculum areas.