In second grade I do a watercolor (watered down tempera paint, that is)
lesson where I introduce the word, 'horizon line' and concept. The
students are given a pencil to draw any type of horizon line (i.e.,
straight, hilly, etc.). We work with two colors this first lesson, green
and blue. With sponges held by a spring type clothes pin, we paint juicy
skies and lush grassy ground AND THE TWO MUST MEET. I encourage big
sweeping arm movements that go from side to side and off the paper.
For the second lesson, I have have over the years, used various add ons
such as collage or thicker tempera paint. For example: a fall tree shape,
color mixing finger print leaves with tempera; on a separate sheet of paper
doing a scarecrow (figure drawing) with color markers, cutting it out and
gluing it to the background (sky, ground, tree picture).
The point is, as precious as those stripes of blue at the top of the page
are, I find most students ready to move beyond this. I don't make a big
deal if, on other projects, they lapse back into symbolic sky strips. By
third grade, with a quick review of horizon line, most of the students are
coloring in the entire sky area. Hope this helps you.