system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do tree trunks have a ring for every year the tree was alive? Why are those rings formed at all?
gpt
When a tree is growing, they tend to do so in patterns of fast growth in the spring/summer and slow growth in the fall/winter. 

The fast growth isn’t as dense, and is a lighter color, while the slow growth is denser and darker. 

There are exceptions to this rule though. Drought/flood/disease or other factors can disrupt the growth cycle.