ast, natural play spaces (e.g., woods, gardens) and playgrounds with unstructured construction materials stimulate diverse and creative play . Fantasy and socialisation are prominent and the social hierarchy is based on the ability to imagine what the space might become rather than on physical prowess . Children experience active play through lifting, pushing, and pulling. They engage in creative, socially interactive play as they construct new structures and play within them . Both in our pilot study [20, 21], and in a study at another Australian school where children played with unstructured materials, reports of injuries, bullying and fighting were almost non-existent .
Sedentary lifestyle and obesity
In Australia over 200,000 children are obese . Childhood obesity is a serious problem with a range of significant medical, psychosocial and economic consequences . Despite the growing awareness of the consequences, the prevalence of childhood overweight and obesity is still high in many westernised and westernising countries [24, 25]. Regular physical activity is associated with both a longer and healthier life , including a decreased risk of obesity , insulin resistance  and metabolic risk . Similarly, physical inactivity during the early years of life is a major contributor to serious medical conditions in children [30, 31]. Time spent participating in sedentary and low energy-expending activities is associated with higher relative weight [32–34] and poorer health. Hence, a reduction in sedentary behaviour is important for promoting health .
Several underlying issues may contribute to the increasingly sedentary lifestyles of some children. In many cases children are reliant on parents when it comes to the activities they undertake. Families live exceedingly busy lives with limited time for children's free play and unstructured activity. To "make the most" of the time they have, many middle-class parents enrol children in organised activity and sport [36, 37] that offer strictly scheduled, structured exercise and play. In families with fewer financial resources, children are more likely to spend more time in front of the television instead . To make sure children's school results do not suffer, there is an increased emphasis on academic tasks, particularly in middle and upper class families. This increased pressure on children, in combination with the perception that there are many "predators" in public spaces, ensures an increase in screen- and homework time and a decrease in time spent outdoors. To save time and keep children "safe," many parents drive them to school and sport--creating a self perpetuating cycle.
Mental ill health and bullying
About 14% of Australian children have significant mental health problems . In children from disadvantaged backgrounds, this figure can rise up to 60% . Play is one of the best ways for children to develop coping skills, and thereby mental health. Play engages children's intrinsic motivation and supports autonomy, competence and relatedness that have been shown to promote psychological wellbeing .
Poor social skills have been linked both to bullying and being bullied  and children with higher self-concept are bullied less often and also are actively included in play more often [42, 43]. Our conjecture is that unstructured materials on the playground will give children who are not very sporty or physically coordinated the chance to play. Teachers participating in our pilot study reported that the unstructured materials promoted play between children who had not played together previously--including children who had formerly been excluded .
Play as an alternative intervention
Play is a universal and profound process that has evolved in both animals and humans. It is spontaneous, exploratory and intrinsically motivated . Children love to explore and delight in opportunities to devise their own free play to experience sheer joy and pleasure in the moment . There are many documented benefits to play  and this study aims to engage children's playful nature to promote greater physical a