system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can the universe be both "infinite" and "expanding"?

Throughout my whole life I've heard that the universe is infinite.  I've also heard that the universe is constantly expanding.  What I don't understand is how it's possible for something to be both infinite and expanding because, to me, the word "expanding" implies that something is finite.
gpt
Imagine a rubber ruler that is infinite in length, that has marks at centimeter intervals in either direction.

This is an infinite ruler, but it can stretch as it is made of rubber.

Now, stretch it. Despite it being infinite the tally marks have moved and are no longer 1cm apart, but a bit further than 1cm. The ruler has expanded, despite being infinite it is now longer than it was before.

That's the general idea behind "expanding universe." it isn't so much that the universe is expanding into something but that the space between two points in the universe are moving apart, the space itself is expanding.