This is a simple exercise that uses positive and negative shapes and lines.
1. Give each student a tall white rectangle, and a black piece of paper that is half the size of the white. The black paper is to sit at the bottom of the white paper. Ask the students to draw three lines to make a smaller black square, the top edge being one side. Have the students cut out the square out in one clean cut (not several pieces).
2. Show how the smaller black square is to flip up, matching corners. Glue down the bottom "negative" shape, and the top "positive" square, leaving a "negative" white space open.
3. Now the students may use a black marker to draw a "positive" vase in the bottom empty square, and a white crayon to draw a "negative" bunch of flowers on the black square above.