28) The Drake Equation

This is an equation that predicts how many intelligent civilisations there might be across the universe. Each letter stands for a different prediction. These parameters are things like: the average rate of star formation in our galaxy, the fraction of those stars that have planets and the length of time for which such civilizations release detectable signals into space. Of course we can’t know the answers to all of them, so the equation is of limited use. But it’s still a pretty cool idea.