The concept of open functions was introduced to capture an intended feature of van Eyck's seminal play sculptures -- they do not suggest a specific action. In a recent study, Van der Schaaf, A. L., S. R. Caljouw, and R. Withagen. (2020. "Are Children Attracted to Play Elements with an Open Function?" "Ecological Psychology" 32: 79-94) aimed to determine the degree of openness of Parkour play elements by asking participants what behavior children will mainly perform on them. In the present study, we used this method to determine the degree of openness of traditional play elements (e.g. a slide), van Eyck's play sculptures, and Parkour play elements. In addition, we measured the time participants needed to answer the question of what action children will mainly perform on the different elements. We found that generally the Parkour elements have the highest degree of openness. Interestingly, van Eyck's play elements appeared not to be that open. Moreover, the reaction time proved to be an alternative measure for the degree of openness.