For play researchers, no one looms larger than Lev Vygotsky. Vygotsky viewed play, particularly pretend play, as a critical part of childhood, allowing a child to stand "a head taller than himself." His biggest theoretical contribution may have been the Zone of Proximal Development: the idea that children are capable of a range of achievement during each stage of their lives. In the right environment, with the right guidance (later dubbed "scaffolding"), children can perform at the top of that range. For instance, Vygotsky explained, when a child can pretend that a broomstick is a horse, he or she is able to separate the object from the symbol. A broom is not a horse, but it's possible to call a broom a horse, and even to pretend to ride it. That ability to think abstractly is a huge mental leap forward, and play can make it happen.