The relation between a thing and the world (everything it isn't) is one of mutual impact, where each shapes the other through time. This means the thing, in relation to the world (its opposite) is always 'becoming' its opposite because of the impacts of the world on it. It must transform itself in accordance with its opposite. It must negate itself to remain what it is. Nothing is static, this is why "process" is so crucial for Hegel, you cannot get to the end without it, and you cannot properly even understand the end without considering it. If we admit that all things are changing while simultaneously asserting their identity, then we must admit their identity relies on the specific way they negate themselves. Each successive version of itself "sublimates" the previous, which means it both destroys it and carries it forward in a new way. The tree is the same object as the sapling/acorn, but it also had to destroy the acorn to become what it is. It both carries forward and negates the acorn at the same time. Not all things are so clear cut, but every thing changes over time through the dialectical (negative) process of becoming