The answer is: When Kirsten turns over paper.
Recycling is the process of converting waste into material or products of potential utility bringing the reducing of the consumption as a consequence. In this scenario, Kirsten turns the paper to take advantage of the blank space. When she gets tired of one side of the painting, she can use the other painting. Kirsten then saved up the use of another paper, using the blank side of the same paper she had used before.