Figure 1 shows the process of lesson study every week, at
Koo Kham Pittayasan School, in which the student as a teaching
practitioner at school, the observing teacher, and the researcher
as a school coordinator collaborated in the design and
teaching planning on Tuesday and participated in observation
based on the teaching schedule assigned from the school director.
They discussed results on Thursday. This process has continued
since 2006 at the school. In the mathematical class, there
was a plan in arranging learning and teaching activities highlighting
the open approach-based problem solving with the aim
that students could participate in activities and show potential
in mathematical thinking with all of their ability. From the researchers’
participation in each step of lesson study and the
continuous observation of elementary student at grade 1 since
the academic year of 2007, it found that while solving problems,
for most of their behavior, the students focused on writing
along with thinking: writing diagram, writing expressions sentences,
and writing description on a self-thinking process with
letters. Students tried to write description of self-thinking by
writing and spelling to communicate with other people (Suriyon
et al., 2011). Rose (1989 cited in Pugalee, 2004) described
the process as “Thinking aloud on paper.” Writing is not only
describing what they think but also providing evidence of
thinking that helps students to be aware of their thoughts and
shows how they solve problems. Writing is thinking evidence
that problem solvers can use to investigate their self-thinking
process. Also, NCTM (2000: p. 61) suggested, “in writing,
mathematics can help students gather their thoughts since writing
requires students to reflect on their work results and clari