The oxalic acid content of spinach has become a major human health concern due to its toxicity. We investigated in this study the residual oxalic acid content in spinach and the free oxalic acid content in the cooking solution. The water temperature and duration of boiling were important factors for the residual oxalic acid content of spinach, while the water volume and salt concentration had no influence. However, the content of free oxalic acid in the cooking solution was influenced by the water volume, salt concentration, water temperature, and duration of boiling. In particular, the oxalic acid content in the cooking solution decreased with increasing salt concentration. Cooking spinach by boiling was demonstrated, and a follow-up questionnaire survey of junior high school students was conducted. The results of the questionnaire survey clarified that there was insufficient understanding of the preparation of vegetables by boiling as studied at the elementary school level. However, most students understood how to boil spinach after the teaching demonstration, and the results of a cluster analysis showed that the students' interest as well as knowledge about spinach had increased