>> astroids are out there and surely one with our name on it, earth's name and small ones will explode and take out regions and cities maybe a transportation grid. the big ones are rarer, render fractions that are rarer and take out the dinosaurs, you get one every 100 million years or so. other things that can take out life on earth are major climate change. if you're a life form that is adapted to a certain configuration or combination of temperature and atmosphere conditions, then if that changes faster than you can respond, that can be devastating that basically your echo system changes and that could be devastating to survival. most life that has gone extinct on earth for reasons repeated to the change in environment. so, we might be the own -- we might be our own source of extinction, if this continues. >> so let's talk about a bit of controversy because you took