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[Display omitted] • The fracture toughness of bamboo increased with the water content. • One function of water in bamboo was to soften the cell wall, which increased the plastic deformation of bamboo and impeded the propagation of cracks. • The other function of water in bamboo was to make the fracture surface smooth, which indicates that cracks propagated easily. • These two functions of water also affected the fracture toughness of other materials. Most engineering materials are chosen based on their fracture toughness rather than their strength. However, water molecules in air would interact with these materials and affect their fracture toughness, especially for the biomass materials. Bamboo is an excellent engineering material, and the effect of water content on its fracture toughness was still unknown. Thus, in this study, the effect of water on bamboo's fracture toughness was investigated, and the mechanism behind it was discussed. The results showed that the growth fracture toughness increased by 7.6 J/m2 per 1 % increase of environmental relative humidity. The effect of the water includes both promoting and impeding crack growth. The hydrated amorphous layer in cell wall impeded the propagation of cracks, while the water layer on cell wall promoted the propagation of cracks. The former effect was the stronger one in bamboo, and thus the fracture toughness increased. This competition mechanism stemmed from water was also found useful to explain the fracture properties of other kinds of materials, such as wood, stone, and clay. Therefore, a reasonable hypothesis to explain the mechanism of water content on fracture toughness of bamboo and even other materials was concluded in this study. [ABSTRACT FROM AUTHOR]