One of the goals of science education is to foster critical and scientific thinking skills. However, if the meaning of the terms "thinking critically" or "thinking scientifically" is not clear, these phrases will become vacuous slogans beyond translation in teaching methods and curriculum materials. Therefore, the fundamental aspect of these terms must be embraced in the discourse of science teaching itself. Since science knowledge is growing in a way that makes it imperative for everyone to know how to handle knowledge itself, the task of getting clear about scientific thinking is imperative. Suggesting that scientific thinking (or critical thinking in science) is more than puzzle-solving and logic-wielding and avoiding a detailed philosophical and psychological treatise on the nature of thinking, some examples of what appears to pass for thinking are presented. These examples show that conventional ways of talking about "thinking" in science education are inadequate to the task of showing what students have to know as base to think scientifically. The nature of scientific knowledge is then considered to build some ideas about what it means to hold scientific knowledge, the essential points illustrated with extracts from science teaching. Implications and recommendations for science instruction are considered. (Author/JN)