The Students' Conjectures: During an archeological dig, the students found a voussoir from a semicircular stone arch. A voussoir is a trapezoidal (wedge-shaped) stone used to build an arch or vault. What are the students trying to find out?

They are trying to find out how many stones there are in the original arch.

Explanation:

Once they've identified the stone, and its main features, they can address what kind of arch was being built there. After this general understanding, the students can predict how many stones were used to raise the arch, and how they were arranged on the structure. The prediction can be proved after all the original structure being totally understood.