The aim of the fourth activity, “Invisible writing”, was to recognize that some of the changes in nature are permanent and some of them are not. Participants first wrote their message onto a sheet with NaOH solutions. When those messages dried, they were mixed within the classroom. The participants brought to light the messages by using given phenolphthalein solutions. They observed that bringing to light the messages was a permanent changing, since none of the solutions could turn it into its invisible stage. Pre-evaluation was a multiple-choice mini-test asking which of the colors should be mixed to make the following colors. They were asked sign “which changing’s are permanent?”