How did the greatest greek warrior of all time lost from a tortoise?

And what you can learn from it?

Ancient Greek polychromatic pottery painting (dating to c. 300 BC) of Achilles during the Trojan War | image: Wikipedia

In Greek philosophy, Achilles or Achilleus was a hero in the Trojan war. He is the greatest warrior of all the greek warriors.

So how was the greatest warrior of all time was defeated by a tortoise, could you believe that? but this actually happened.