Are we alone in the universe? What are cosmic rays? Why is it that one sugar in coffee is never enough?
These are the simple questions that have defined how we live our lives, and indeed will carry on defining them until we get them answered.
With astronomers spending their every waking moments staring and listening deep into space the hope is that eventually, we'll get a sign that in fact we aren't the only people in the universe who like sugar in our hot drinks.
Until that point however, here are the 10 greatest mysteries that remain an absolute mystery to science.