We've found that some students believe that the approximation that their calculators show for irrational numbers is exact. Here's an activity that you can advertise as long multiplication review. After they study our calculations have a discussion just to clarify what they know and bring home the notion of approximations.
The activity: squaring-square-roots.pdf
For members we have an editable Word docx and teaching hints.
CCSS: 5.NBT.7, 6.NS.3, 7.NS.3, 8.NS.1