Some extractives are surface-active substances and unavoidably affect the floor properties of paper, corresponding to water adsorption, friction and strength. Lipophilic extractives usually give rise to sticky deposits throughout kraft pulping and should depart spots on paper. Extractives also account for paper odor, which is important when making food contact materials. There is a robust relationship between the properties of wood and the properties of the actual tree that yielded it. The density of a wooden correlates with its strength .