This is a very arguable question. As Martin explains in the answer, the Order aggregates the Product. And this can be considered true. Grady Booch in his "Object-Oriented Analysis and Design" brings a similar example for association - a sale is associated with products in that sale, and vice versa. And a sale doesn't aggregate products. So all examples should be domain-specific, because from another point of view the association may become more specific. Another example is the composition of Documents using Paragraphs.
So everything in this field strongly depends on the context. This is the OOP.
You can try to apply your knowledge to a particular project you are going to design. I would recommend you to read Grady Booch's book, if you haven't done it yet. Lots of books have been written since it, but it is still the Bible of OO*.
