system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do children build forts?

Why do children always feel the need to build forts? It seems that children do not discuss forts as vital to their existence. But many children make them without the knowledge that other children make them as well. Is building a home for ourselves in our nature even as children? Does this have anything to do with nesting?
gpt
Many aspects of play behaviour are mimicking adult behaviours. Children who built forts, play homes, play schools, or play workplaces are in many ways mimicking what they see adults doing. Part of play behaviour in mammals is for the young to develop and train the physical and cognitive aspects that will enable them to function as adults. In order to do this they must first practice these adult skills through play. 

There are other aspects of play that may overlap or may be completely separate from mimicking adult behaviour. These include, but are not limited to: strengthening the body, understanding physical limitations, setting up peer and social groups, learning how to be a part of a social network, learning how to resolve disputes, or setting up a power structure. All mammals engage in play, and for reasons quite similar or exactly similar to our own in some cases.