I want to test the ability of students to read a passage and answer questions about it. Could you please come up with a good question for the passage "Wood is a heterogeneous, hygroscopic, cellular and anisotropic material. It consists of cells, and the cell walls are composed of micro-fibrils of cellulose (40% – 50%) and hemicellulose (15% – 25%) impregnated with lignin (15% – 30%)."? 

The heterogenous and hygroscopic material is made of?