A tree is a hierarchy — one root, branches that never rejoin, leaves at the ends. Three places where trees quietly organize the world around us.
A filesystem is a tree. Click any folder to open or close it; the tree relays itself around your choice.
When a browser reads HTML, it builds a tree. Hover anything on the left — watch the corresponding node light up on the right.
Arithmetic as a tree. Operators at the branches, numbers at the leaves. Evaluating means walking up from the leaves — children first, parent after.