Guided by the Boundary Activity based Learning (BABL) principle, mobile technology-supported inquiry learning activities were implemented in a primary four science class in Hong Kong. An exploratory study was conducted to examine the effects of the BABL guided inquiry activities on students' learning performance and to explore how the key element, the boundary object, operated in different learning spaces. In the study, mixed research methods were used to evaluate students' conceptual understanding and their engagement in and attitudes toward BABL activities. The reciprocal interactions of students' cognition were qualitatively analyzed in terms of the forms and functions of boundary objects in the BABL environment. The results showed that students made significant improvements in conceptual understanding and were engaged in BABL activities. The study also revealed that the generation of abstract boundary objects, together with physical boundary objects, promoted students' learning and thinking as they shuttled between the classroom and the outside. This research contributes to informing educators about how to design and implement technology-supported teaching and learning through the use of boundary objects in crossing learning contexts. Copyright © 2020 International Forum of Educational Technology & Society.