Evidence continues to accrue around the difficulties people have understanding new and complex visualizations, which in turn provides continued incentive to explore additional methods of supporting visualization viewers. Through leveraging viewers’ spontaneous visualization sensemaking activities, we introduce a Serendipitous Explanation Approach (SEA). A significant part of SEA’s contribution is making active use of a viewer’s spontaneous, exploration interactions to offer in situ explanations. SEA adds semantic and structural explanations, making use of visual transformations, and additional representations of visualization elements themselves to communicate meaning through highlights, reconfigurations, and in-situ annotations. We designed and studied VisTips as an instantiation of SEA. Our study demonstrates the prevalence of spontaneous interaction and appreciation of this approach. Among the explanation types, visual transformations stood out as especially impactful. Our findings also offer practical insights for future use of the ideas in SEA, opening possibilities for seamlessly integrating explanatory visuals that support viewers’ natural visualization sensemaking.