Science requires imagination nourished by knowledge, experience and sustained critical thinking. Science teaching has the same requirements, but metacognition is even more important to a teacher than it is to a practitoner of science. Critical thinking is essential to both science and science teaching: in either domain, imagination relies on sustained critical thinking based on relevant knowledge. Knowledge can be acquired by the prepared mind, but the capacity to think must be nurtured and exercised. The raw material of critical thinking is comprehension of fundamental processes rather than knowledge of facts. In parallel with the rapid growth of scientific knowledge, there has been a tendency to focus on content rather than developing the skills needed to do science. In essence, what has been neglected is critical thinking.