Mobility data: it’s everywhere and it reveals what we do, where we go, how long we stay, and who we interact with. Researchers at Stanford used that data to build an incredibly powerful model of COVID surge and spread, one that shows both how the small decisions we make about staying home matter and the terrible consequences of taking no action. We spoke with Serina Chang of Stanford University. Serina is one of the researchers on this project and the first author of the recent paper that lays out its findings. We spoke to her about how policymakers can use her work to make hard choices about reopening and — the question on everyone’s mind — what to do about Thanksgiving.