This research is to investigate the effectiveness of learning materials and lessons plan to provide understandings of separating mixtures by using STEM education approach. Furthermore, this is to define creativity, critical thinking, and metacognition of students. In these lessons, the participants were 10 middle school students divided into three groups. Students were asked to separate the mixtures (sphered styrofoam particles and plastic balls with different sizes) by using tools that made by students. Students had to design tools from plastic bottles. Students were given opportunities to try their products, and then redesign it. In order to collect data, students had to draw their designs in the worksheets. The results showed that students failed in the first attempt, because they did not consider the different sizes of mixtures. However, in the second attempt, they started to design tools that they considered the sizes of the mixtures. Due to the time limitation, they could not separate them completely. Their ideas and strategies will change after they develop and try their product. Therefore, these lessons provide students to improve their 21^st century skills.