Why do trees have rings inside their trunks?

Answer: Wood grows fast in spring and slower in late summer

Wood grows fast in spring and slower in late summer ✓ — In spring a tree grows fast, making wide, pale cells; in late summer growth slows and the cells turn small and dense. That sharp change from dense wood back to pale wood each year draws a ring, so counting rings tells a tree's age.

Each year's old bark gets folded inside the trunk — Bark doesn't fold inward; a thin layer of growing cells under the bark adds new wood on its inside and new bark on its outside. The rings are layers of wood from different seasons.

Winter storms crack the wood into layers — Storms don't make rings, and trees in sheltered spots have them too. The rings come from the yearly rhythm of fast spring growth and slow late-summer growth.