A strip of paper placed on a hand spontaneously curls upwards. This simple observation illustrates the ability of a relatively homogeneous hygroscopic structural material, paper,
to sense and respond to the microclimate near a non-equilibrium system, a moist evaporative
boundary layer. We quantify this interaction using a simple experiment and show that it can be
understood in terms of a minimal model. A small modification of this paper hygrometer that
makes one or another surface partly hydrophobic using a crayon or tape allows us to create a
hygro-oscillator or a hygromotor that converts transverse moisture gradients into lateral oscillations or directed motion. Our study shows how treating paper as a responsive structural material
allows us to extract information and work from a microclimatic boundary layer, transforming a
messenger to a machine.