Many of the psychological benefits of outdoor play are already well established. Our brains evolved in natural landscapes, and our perceptual(感知的)systems are particularly well suited to wild outdoor spaces. 

This means that natural scenes provide the perfect level of stimulation, which is thought to help recharge the brain when it is tired and easily distractible. Supporting this theory, one study found that children with attention-deficit hyperactivity disorder (ADHD) were better able to concentrate following a 20-minute walk in the park, compared to a 20-minute walk on the streets of a well-kept urban area. Being exposed to grass and trees seemed to have had a beneficial effect on their minds. The authors recommended using such "doses(剂量)of nature" as a safe and accessible way of supporting children with ADHD, alongside other tools.

Besides these restorative effects, outdoor play can offer valuable learning experiences. For example, playing with mud or sand can help children develop the way their senses and movement interact, which allows the child to gradually understand his or her body signals. 

Such activities — away from the house or classroom — may also help children to find ways to cope with emotions that maybe hard to explore in other environments. So-called "sand tray therapy", which involves using sand and clay to express one's thoughts and feelings, is an accepted form of counselling for children who are struggling to verbalise their emotional state.

When it comes to the child's physical health, the most obvious advantage of outdoor play maybe the exercise A child may find it easier to buildup strength and endurance in a large open space resulting in a reduced risk of overweight, according to one study led by Elizabeth Gershoff, professor of human development and family sciences at the University of Texas at Austin, US.

The latest findings, however, suggest there could be a host of other advantages to playing in natural environments — and the secret maybe alive in the mud itself.