Spoiler alert: the answer is Carleson measures, named after Lennart Carleson who introduced them in 1958 in order to solve a problem in analytic interpolation. Carleson measures have since become a critical tool in harmonic analysis, playing a fundamental role in the study of singular integral operators such as the Hilbert or Riesz transforms, through their connection with BMO, the John-Nirenberg space of functions of bounded mean oscillation. These measures also play a special role in the theory of regularity of solutions to second order divergence form elliptic/parabolic operators, and this in turn has led to new ways to quantify geometric smoothness of lower dimensional sets in Rn. The lecture starts with an introduction to three major theorems in analysis/PDE 1958-1964, whose incredible connections were not obvious at that time.