A teachers\' association can play an important role in the development of teachers\'
professionalism. Unlike the obligatory in-service education such, as the certificate education, the
teachers\' association can provide a festival where in-service teachers develop their
professionalism effectively, when the members of the association voluntarily participate in the
festival. The teachers\' association, \"Teachers for Exciting Science\" has a meeting every week at
Hanyang University to share science experiments. In December, 2007, they opened a new
festival where the experiments were presented altogether. Thirty-one experiments were
presented and the participants experienced and discussed the experiments. We analyzed the
interactions among teachers participating in this festival, through which we described the
concrete process how the teachers\' association contributed to the science teachers\' professional
development. As results, we identified four interaction patterns. First, science teachers
transferred the experiment skill by demonstration, and embodied the skill by following the
demonstration. The specific guidance from the experienced teacher could contribute to the
improvement of the experiment skill directly in the experiment where a hands-on activity is
important. Second, science teachers exchanged practical information to implement the
experiment in the school setting. They shared the name of new experimental materials and the
method to get the materials. In addition, teachers videotaped the experiment process and asked
for the video clips of the experiments. Third, science teachers could improve the experiment
through in-depth discussion on the experiments. They constructed an ample repertoire on the
experiment by new ideas on the experiment methods, theoretical explanations, and discussion
on the effect of the experiments. Fourth, science teachers promoted the professional
development by the emotional interactions. They shared the enjoyment of the novel experiment
results, and raised interest in the science. In addition, we could find the support for the
professional development of female science teachers through the education of their children and
sharing responsibility over them.