stroke of genius, a

A particularly inspired, clever, or innovative act, idea, or decision. Their decision to let customers from any other operating system migrate their information was a stroke of genius. I must say, it was a stroke of genius getting the kids all those puzzles—they've been playing quietly for hours!

an act of genius; a very clever and innovative idea or task. Your idea of painting the rock wall red was a stroke of genius.

A brilliant idea. This cliché, in which stroke betokens the sudden appearance of the idea, dates from the second half of the 1800s and at first was always used admiringly. Thus the Tacoma News of December 13, 1889, stated, “The latest nickel-in-the slot scheme is really a stroke of genius and is destined to revolutionize cheap literature in this country.” Later, however, the term was often used sarcastically, as in “You forgot the tickets? That was a stroke of genius.”