"Nails in a Jar" is a formative assessment from Page Keeley's Uncovering Student Ideas in Science, Volume 4. Studens are asked whether the mass of a sealed jar of wet nails would change as the nails rusted. The probe elicits student thinking about both chemical changes and conservation of mass in a closed system. Note: the details of oxidation and the term mass are a secondary level expectations. The term weight may be substituted for better alignment to elementary expectations.