"A great t-shirt gift idea for any fan of science, physics, astoronoy anything to do with the stars. The Drake Equation, is used to estimate the number of communicating civilizations in the universe, or more simply put, the odds of finding intelligent life in the universe. N = the number of civilizations in our galaxy with which communication might be possible (i.e. which are on our current past light cone);
R* = the average rate of star formation in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fl = the fraction of planets that could support life that actually develop life at some point
fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space"