What Does "Natural" Mean?

I have found it fascinating over the years when I hear people use the term "unnatural" when referring to human things. How are they unnatural? Unnatural means that something is not related to Nature. So...how exactly are our things not natural? Are we not a part of Nature? We evolved as a part of this Earth just as the birds and bees. All of our products are made from products of this planet so I must think of them as "natural". Why are our houses less natural than a bird's nest? Both are typicall framed with products from trees. Our glues are similar to bird's saliva products. Both are a reformatting of the Earth's natural resources. Seems natural to me!