Columbus' egg

Columbus' egg  refers to a brilliant idea or discovery that seems simple or easy after the fact. The expression refers to an apocryphal story, dating from at least the 15th century, in which it is said that Christopher Columbus, having been told that finding a new trade route was inevitable and no great accomplishment, challenges his critics to make an egg stand on its tip. After his challengers give up, Columbus does it himself by tapping the egg on the table to flatten its tip.

https://en.wikipedia.org/wiki/Egg_of_Columbus