Recently, many researchers had reported their work about sub- and superluminal propagation. And several experiments had demonstrated the signal evolution of slow and fast light. In this letter, the authors described a simple experiment for the tracing of the light signal in saturated absorption (SA) and reverse saturated absorption (RSA) media, firstly. We had, directly, observed the evolution of slow and fast signals with different waveforms that traveled in a ruby crystal and C60. Through tracking the signal when slow light (v g ≪ c) and fast light (v g<0) was observed in a ruby crystal and C60, we concluded that asymmetrical absorption on the periodical signal could account for the generation of slow and fast light in SA and RSA media.