Student Deeply Consider what Conservation of Mass means. This was a tough "get" for us. Students had to think about what variables were involved in determining if a chemical reaction changes the mass of the products relative to the starting reactants. For them, I could see they were having trouble focusing on two things at once.
It was questioning and probing that took them into realizing they had to keep both things going at once. As soon as they had that, they were much more able to define the variables they'd study and map out an investigation plan.