Task: Question Generation
Input (English): In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place. This firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow.
Output (English): What property of knots does the amount of time a branch stayed on a tree while the stem kept growing affect?