For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some 'organic element' since I couldn't accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
This one makes Fermi's Paradox far more confusing and terrifying to me. The time it took to go from agriculture to the steam engine is nothing compared to the age of the universe, absolutely nothing, and from the steam engine to modern technology is fuck ton nothing.
An intelligent species could go from stone age technology to nuclear weapons in the blink of an eye.
And that's just life as we understand it. We have no idea if we're the equivalent of Flatland in a higher spatial dimension or something. There could be stars with entire civilizations of plasma-based intelligent life churning inside of them. There could be intelligent civilizations lurking in each and every single subatomic particle.
It's possible no matter how far out or far in we look, we just keep finding more universe, more space for something to inhabit, forever...
As they said on
, if we look everywhere and we're the only intelligent species anywhere in this universe ... well that would be weirder than if life is hiding all over the universe.