In an effort to increase our understanding of what mathematics young children are capable of when provided with playful yet purposeful opportunities to learn, Canadian researchers initiated lesson study over 4 years with teams of teachers of young children (ages 4-7). The lesson study process followed a lesson study cycle of goal setting, planning, implementation and reflection, with one major variation: a focus on the design and implementation of exploratory tasks. Exploratory tasks are brief, collaboratively planned activities that focus on precise mathematics content with engaging contexts. The lesson study team featured in this paper presented students with challenging exploratory tasks to investigate length, perimeter and area measurement concepts as they progressed toward the public research lesson. Through the process, lesson study teachers increased their abilities to look and listen for student mathematics thinking that affected their subsequent planning to tasks. Notably, teacher estimations of what young children can do were expanded and raised. Students also made significant gains on pre-post interviews, suggesting that exploratory tasks are a powerful variation of lesson study that increases teacher and student learning due to increases in frequency, emphasis and implementation of quality tasks.