One of the most important aspects in enjoying fantastic media is a liberal suspension of disbelief. Superman can fly, ET can make a space phone out of random household objects, and Jedi can use lightsabers without constantly cutting their arms off because they just can, and audiences, for the most part, accept these as facts.
However, some fans like to get into the specifics and try to figure out just how their favorite characters can do what they do. One of the biggest mysteries to keep these die-hards up at night is how Captain America miraculously survived being frozen in the arctic for over 70 years.
Though the specifics change from continuity to continuity, the general premise is always the same: in the waning days of WWII, Steve Rogers heroically sacrificed himself by flying a jet/rocket/missile into the frozen waters of the north Atlantic, where he was cryogenically frozen until the Avengers/SHIELD found and thawed him out.
For the most part, comic readers and film audiences have simply accepted that it was possible for a super soldier to be frozen for seven straight decades and come out alive. However, a redditor recently revealed that Marvel has actually supplied an official explanation at the Avengers S.T.A.T.I.O.N., an interactive MCU exhibit in Las Vegas.
According to the exhibit, a blood test after he was thawed out exposed the secret to Cap's survival: while he had been frozen, his blood was fine. By raising his body's glucose levels, Steve's blood lowered his body's freezing temperature, creating a 'cryoprotectant' similar to those of hibernating bears and frogs.
Because the water literally couldn't freeze him, Steve was just stuck in suspended animation, comatose for the better part of the 20th century. This revelation puts to bed a query fans have had since Captain America was reintroduced with the first incarnation of this backstory in 1964.