In pump-probe reflectivity, two ultrafast laser pulses separated by a controlled time delay are directed onto a sample. The first pulse, the pump, transiently excites the material under study (in the same way a hammer excites a bell to ring). The second pulse, the probe, is then used to measure the reflectivity of the material. By recording how the reflectivity changes with time after the pump pulse (much like how our ears hear the tone of the bell diminish after it rings), one can learn about the dynamical properties of the material on femtosecond timescales.