Friday's was apparently too easy -- Sam [almost] knocked it out nearly as soon as I posted it. So let's try another:
Draw a square with horizontal and vertical sides. Draw a diagonal from the lower left corner to the upper right corner. Draw another line from the lower right corner to the midpoint of the top side.
Consider the triangle formed by these two lines and the bottom side of the square. What portion of the area of the square is in that triangle?