Introduction The flipped has gained prominence in recent years. There is no uniform definition of it. The flipped in this paper refers to the pedagogical design that inverts the teacher's instruction in the out of formal class time and uses class time for students to actively engage in practice and knowledge construction with technology support (Baepler, Walker, & Driessen, 2014). A review of the literature shows that the approach adopted, in many cases, intends to make use of videos as lecture instruction out of class time, thus affords more time to engage students in active (e.g., Charles-Ogan & Williams, 2015; Chen, Yang, & Hsiao, 2015; Jungic, Kaur, Mulholland, & Xin, 2015; Moore, Gillett, & Steele, 2014; Muir & Geiger, 2015; Sohrabi & Iraj, 2016; Wasserman, Quint, Norris, & Carr, 2015). However, it appears that the way of instruction (direct instruction) remains unchanged except that the time spent on the lecturing in class is performed at home. How to enhance students' conceptual understanding and develop their problem solving skills in the design and implementation of the flipped has rarely been addressed. How to flip the classroom? The paper reports quasi-experimental study comparing the traditional flipped classroom pedagogical design to the productive pedagogical design in the flipped for 2- week curricular unit on polynomials in Hong Kong Secondary school. Productive failure is defined as a design that affords students opportunities to generate representations and solutions to novel problem that targets they have not learned yet, followed by consolidation and knowledge assembly where they learn the targeted concept (Kapur, 2015, p. 52). In the productive pedagogical design, students explored, discussed, and solved problems related to the new concepts first in class even though they might come across failures, followed by consolidating the concepts and associated procedures using video clips at home supported by mobile devices. The pedagogical design is referred to as productive failure-based flipped classroom in this study. The rest of the paper reviews the literature, followed by research methods. Then the results are presented and discussed. Relevant literature Traditional flipped classroom pedagogical design The flipped is also termed as inverted classroom or blended learning with various definitions (Chen, Wang, Kinshuk, & Chen, 2014). In general, the flipped attempts to free student class time from lectures by providing the new instructional content (including concepts) in the form of video-clips for students to watch as homework; then use class time for active where the teacher acts as facilitator to organize class activities to deepen their conceptual understanding (Roehl, Reddy, & Shannon, 2013). This type of pedagogical design is referred to as traditional flipped classroom and is used interchangeably with the flipped in this paper. Active is known as any instructional method that engages students in their process through collaborative and problem-based activities to develop their critical thinking and problem solving skills (Prince, 2004). Although teacher-student contact hours do not change, students can re-play the instructional content at home, and have more teacher-student engagement in class. The ultimate goal is to shift teacher-centered instruction to student-centered to change the role of the teacher from sage on the stage to guide on the side (King, 1993). The flipped is gaining popularity. Previous studies have typically been conducted in higher education settings (e.g., Abeysekera & Dawson, 2015), and in recent years, research on the flipped in school education is on the rise, particularly in mathematics and teaching (Muir & Geiger, 2015). …