A chart showing the explosive expansion in proportion of Granzyme K+ CD8+ T cells in mice across multiple organs as they age (Cao lab) If you looked at two snapshots of the same maple tree taken in July and December, you’d see a dramatic change from summer’s full green crown to winter’s bare branches. What those two photos don’t show you, however, is how the change occurred—gradually or all at once? In truth, deciduous trees tend to hold out for an environmental signal—a change in light or tempe...