There is a mystery to be solved! This lesson plan asks students to identify the Who, What, When, Where, Why, and How of ocean acidification (OA). Global oceans have absorbed approximately a third of the CO₂ produced by human activities, such as burning of fossil fuels, over the past decade (Sabine et al. 2004). This accumulation of CO₂ in the ocean has lowered average global ocean pH and decreased the concentration of carbonate ions (CO³_2-) (Fabry et al. 2008). As a result of this OA, the carbonate chemistry of the global ocean is rapidly changing and affecting marine organisms (Orr et al. 2005). Pteropods (open-ocean snails) are considered bioindicators of OA due to the vulnerability of their aragonitic shells dissolving under increasingly acidic conditions from a changing climate (Figure 1) (Orr et al. 2005; Bednaršek et al. 2014). This lesson plan can be found at: >https://www.vims.edu/research/units/centerspartners/map/education/profdev/VASEA/lessons.php.