system
Given the following post, respond as if you're commenting on Reddit:
human
How do tree rings form?
gpt
When the Treestump grows, it becomes thicker too. Now because that takes a long time this process has to happen throughout a lot of seasons.
In winter there‘s less energy for the chemical reaction that is used to grow wood, which leads to a slower growing process, this is where you can see a dark „ring“ on a tree stump. 
That is also the reason why counting the rings lets you determine a trees age. It basically lets you count the amount of winters that the tree has grown through! Im sorry i cant go further into the chemical process, but if youre interested there are probably a lot of articles on the internet about wood and treestumps that can explain it better than me :)