system
Given the following post, respond as if you're commenting on Reddit:
human
If the universe is infinite, what do people mean when they say it keeps expanding?

Is it just a philosophical question?
gpt
Firstly, infinitely large things can expand. Imagine an infinitely long rubber band with sharpie marks every inch. Then it stretches so that the sharpie marks now occur at 2-inch intervals. The 1" mark stretches to where the 2" mark used to be; the 2" mark to where the 4" mark used to be, and so on.  This is, in fact, the sense in which an infinite universe expands.