| 1 | The Child's Conception of Space, Jean Piaget, Bärbel Inhelder, Psychology Press, 1997. |
| 2 | Children's Playground Design, Nor Hadilah Mohd. Hanapiah, Universiti Teknologi Malaysia, 2009 |
| 3 | Early Childhood Playgrounds: Planning an Outside Learning Environment, Prue Walsh, Routledge, 2016 |