Every second, 65 billion neutrinos from the Sun pass through every square centimetre of your body.  They do you no harm - they don't even notice you (indeed, they don't notice the entire Earth: almost all of those 65 billion will still pass through you as you sleep, having gone through the Earth to do so).  Almost massless, almost devoid of any interaction with other matter, neutrinos would seem to be as close to nothing at all as it's possible for matter to get.

And yet, if neutrinos didn't exist, the Sun wouldn't shine, nearly all the elements of the periodic table would not exist in nature, and quite possibly the Universe as we know it wouldn't exist either.  In this talk, I'll try to explain how neutrinos were discovered, how we study them today, and the many roles that they play in physics and astronomy.