Besides physically, I guess humans are pretty much semi-solid mentally because humans undergo changes constantly.
The person you are yesterday is not the same as the person you are in this moment. Our brains are constantly adapting and changing to be better, to evolve, to survive. Like when you look back on your Facebook posts and have a massive cringe attack whenever you encounter an embarrassing status/photo from the past, THAT'S AN INDICATOR THAT YOU HAVE CHANGED. So, my theory is, if we are really solid/rock hard, we won't be able to accept other good or bad elements that enables revolution and evolution.
Haha this is all just a random thought. Maybe I shall write a thesis on this topic in the future but for now I don't knowwwwww
P.S To those science people out there, I'm sorry if I sound stupid hahaha