Video from The Thomistic Institute
"How can the water in a river always be flowing by and yet it remains the same river? By wrestling with this and various puzzles regarding change, Aristotle discovered the distinction between act and potency or actuality and potentiality. Aristotle realized that all the things in nature are a blend of act and potency, like the water. For example, an acorn is potentially a fully-grown oak tree; a child is potentially a grown man; and, the water in the ocean is potentially a rain cloud over land. Aristotle realized that change consists of the actualization of the potentiality latent within the things of nature. Act and Potency (Aquinas 101) - Fr. James Brent, O.P." from video introduction.