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Imagine trying to understand the follow equation for Frank Drake's Fermi Paradox which estimates the number of technological civilizations that might exist among the stars.  \begin{equation}\nonumber  N =\underbrace{\overbrace{R}^{\approx10} \times \overbrace{f_p}^{\approx 1} \times \overbrace{n_e}^{\approx 0.2}}_{\sim 2} \times \underbrace{f_l \times f_i \times f_c \times L}_{?}  \end{equation} \begin{itemize}  \item $R$ the rate of star formation, which tells how many stars are born every year in our Galaxy.  \item $n_e$ is the average number of habitable planets in any planetary system  \item $f_p$ is the fraction of stars that have planets   \item $f_l$ is the fraction of habitable planets that host life  \item $f_i$ is the fraction of life-bearing planets that develop an intelligent life-form  \item $f_c$ is the fraction of intelligent life-forms that decide to communicate  \end{itemize}