How do you define "nature?" If we define it as that which is untouched by humans, we won't have any left, says environmental writer Emma Marris. She urges us to consider a new definition of nature -- one that includes not only pristine wilderness but also the wild, untended patches of weeds growing in urban spaces -- and encourages us to bring our children out to touch and tinker with it, so that one day they might love and protect it.