system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes trees to grow rings and how are they exactly a year old?
gpt
There's a lot of good answers here, but the simple ELI5 answer is that the color of growth in the tree depends on how fast it is growing. In the winter, trees grow slowly, which creates the dark part of the ring. In the summer, trees grow more quickly which creates the light part of the ring. 

Because of this pattern, each ring marks the passing of the seasons, and thus a year.