Why do trees have rings inside their trunks?

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

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.

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.