"You can't step into the same river twice!"

Heraclitus was an ancient Greek philospher from around 500 BCE, and his big idea was "everything is always changing." He's famous for saying, "You can't step into the same river twice," because by the time you step in again, the water has already flowed away-just like everything in life keeps moving and evolving.

He also beieved that opposites(like day and night, life and death) are connected and that conflict is what keeps the world going. He thought fire was the basic element of the universe, symbolizing constant change.

Basically, he was the guy who looked at the world and said, "Nothing stays the same, everything is in flux."