In 1885, he conducted experiments on himself to understand how long the human mind retains information over time [15]. The experimental results can be plotted on a graph what is now known as “Ebbinghaus forgetting curve” (see Figure 1). Ebbinghaus discovered that a human being is apt to rapidly forget the knowledge he has just learned from some material, but review of the material over time makes the knowledge more stable in memory. Apparently, reviewing the material will recall the corresponding knowledge at the same time. This extrapolated the hypothesis of the exponential nature of human forgetting and the relationship between recalling and forgetting mechanisms.