Full Idea
The form of a statue depends upon bronze (or some similar stuff) for its existence, while the bronze has no comparable need for the form of the statue. The bronze can exist before acquiring the form, and continue after the form has been removed.
Gist of Idea
Statues depend on their bronze, but bronze doesn't depend on statues
Source
report of Aristotle (Metaphysics [c.324 BCE], Z.3) by Mary Louise Gill - Aristotle on Substance Ch.1
Book Reference
Gill,Mary Louise: 'Aristotle on Substance: Paradox of Unity' [Princeton 1989], p.39
A Reaction
Some would cite this as precisely the modal difference between them that seems to suggest they are two objects. I would say that their different status shows that they shouldn't be thought of as two 'objects'. An object with two natures?