You’ve been lied to your whole life.
You know how when you buy meat you’ve seen that red liquid that generally comes with it, and you’ve obviously just assumed (and been told) it was blood?
Well, guess what? IT’S NOT BLOOD!
That red liquid is actually a protein called myoglobin. Its function is to store oxygen in muscle cells.
And it’s this protein that “colors” the meat red and consequently also gives that same color to the liquid (which we mistake for blood).
When you cook meat, the myoglobin changes its color from red to brown, and that’s how you know the meat is cooked.
So, now you know.