How did the first person that went to space know that they had to wear a spacesuit?

Hank: There's a couple of things we need to realize in order to know that a person that went to space had to wear a spacesuit.  The first one is that air is stuff.  It's made out of particles and it has mass.  It's matter, and that took us a pretty long time to figure out, actually.  The second is that because, you know, you could just do it from that and the logic would be it has matter, and thus it is affected by gravity and thus it is pulled down toward the Earth and thus as you get further away from the Earth, there will be less of it because of all the matter of the atmosphere we have is pooling toward the gravitational center of the Earth.  

We figured that out, though, rather than sort of guessing at it, just by going up on to mountains and seeing that the pressure of the air went down, so there was less air around and so you can just sort of draw a sort of line from that to say like, as you go higher up mountains, pressure goes down, eventually, it's a vacuum.