Walking is the main form of locomotion on land, distinguished from running and crawling. When carried out in shallow waters, it is usually described as wading and when performed over a steeply rising object or an obstacle it becomes scrambling or climbing. The word walking is derived from the Old English walkan (to roll).
Walking is generally distinguished from running in that only one foot at a time leaves contact with the ground: for humans and other bipeds running begins when both feet are off the ground with each step. (This distinction has the status of a formal requirement in competitive walking events, often resulting in disqualification even at the Olympic level.)
The average human child achieves independent walking ability between nine and fifteen months old.