system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What keeps time moving forward?

As I understand it, we're in a 4-dimensional universe, where time is one of the dimensions. Although the spatial dimensions are constant, time always keeps moving forward. Why is that?
gpt
What is time? Can you fill a cup with it? Can you measure out a piece of time and give it to someone? How do you know time has passed at all?

The easiest to understand description of time I've ever gotten is: change. You perceive time because things change. Simple observable things like the sun moving across the sky.

But also very complex things like the way you age. Atoms decaying. All the chemical reactions that make up existence going about their way. Using up elements, creating new elements from those reactions.

If nothing happened, if nothing changed in any way, if the universe was frozen in a state of sameness... would time still be passing? Not in your perception.

So why does time only move in one direction (or at least why do you perceive time to only move in one direction)? Well have you ever made a cup of coffee? Boil the water while steam escapes. Percolate the hot water through the coffee and the filter, mixed in the milk, dissolved the sugar, drank the coffee. Have the drink go through your digestive system and pee out the waste.

It's easy to do but hard to undo. You can experience that sequence of events in one direction but you can't undo it to arrive at the original state where the coffee is in the jar, the water is in the machine, the sugar is in the pot and the milk is in it's cup.

Time is like that. You experience it because you witness sequential change. But those changes can't be undone.