Context: There is a strong relationship between the properties of wood and the properties of the particular tree that yielded it. The density of wood varies with species. The density of a wood correlates with its strength (mechanical properties). For example, mahogany is a medium-dense hardwood that is excellent for fine furniture crafting, whereas balsa is light, making it useful for model building. One of the densest woods is black ironwood.

Question: What kind of relationship exists between a tree's properties and its wood's properties?
Answer: strong

Question: What property of wood varies according to species?
Answer: density

Question: What property of wood has a correlation to its density?
Answer: strength

Question: Is mahogany a hardwood or a softwood?
Answer: hardwood

Question: What hobby is balsa wood often used for?
Answer: model building