In the morning, before the sun came out, a group of Nigerian-Igbo children, boys, and girls, ages four through six, would gather in the front yard of the family house, and we would use a dry twig to scratch out a grid of columns and rows inside a six-foot square area on the sandy, earthy ground. Then we would take turns and line up with our backs to the grid. From outside the margin, the first child would throw a pebble over his head, hoping that it would land in any of the smaller units of the grid. Then, still in front of the margin of the grid, the thrower must retrieve the pebble, wherever it landed, from outside the margin.
A good throw was when the pebble landed with a thud in the center of a unit, where the child could lean on one leg and one hand, stretch his body and retrieve it with the free hand. A successful throw and retrieval gave the child ownership of the unit, and he could use the acquired units to retrieve future thrown pebbles. The most accessible units to get were those closest to the margins, and we would attempt those first. A pebble that did not fall inside any unit was a bad throw, allowing the next child to throw.
Aiming and throwing overhead with the back turned was challenging to most of the children. It was like groping in the dark. One had to mentally calculate the position of each of the units so that a thrown pebble could fall into them without skidding.
There was a lot of motor planning (praxis) that went into that critical part of the play. For example, the child had to think of how much force to apply to the pebble, and in which direction he wanted it to go. He also had to remember the units that were still open; that is, those units not already acquired. Retrieving the pebble from where it landed was also a challenge. To do that, they had to support their body on one leg and one arm, while using the other hand to pick up the pebble. This maneuver must have immensely tasked the vestibular system of balance, as well as the joints and the proprioception. Children who did not have a sound balancing system would often tip over and fall flat on their abdomen.