Piece of Cake
We use this idiom to mean that something is very easy. That particular activity we are talking about represents no challenge at all.
Cake is a dessert that gives us all pleasure. Nobody would dare to reject a piece of cake. This phrase is was coined in the United States during the XX century. The poet and humorist Ogden Nash’s Primrose Path wrote in 1936: “Her picture’s in the papers now, And life’s a piece of cake.”
Other similar expressions are “As easy as pie, A cake walk, that takes the cake/biscuit, Mickey Mouse stuff, kindergarten homework.”