easy as pie
Also, easy as falling or rolling off a log. Capable of being accomplished with no difficulty, as in This crossword puzzle is easy as pie. The first term presumably alludes to consuming pie (since making pie requires both effort and expertise). The variants most likely allude to standing on a log that is moving downstream, a feat in which falling off is a lot easier than remaining upright. Mark Twain had it in A Connecticut Yankee in King Arthur's Court (1889): "I could do it as easy as rolling off a log." The first colloquial term dates from the early 1900s, the colloquial variants from the 1830s. For a synonym, see piece of cake.