This is the root of the Interactive Primer — the single point from which the whole tree of knowledge grows. Everything else is reached by climbing from here, following each concept's prerequisites upward.

Three good places to start:

• Counting — the very first idea, the root of arithmetic. From here you can climb to addition, multiplication, fractions and beyond.
• The Pythagorean Theorem — a taste of where the tree leads: a classic result of geometry, with an animated proof.
• Welcome, Instructors — if you'd like to build lessons rather than learn from them, start here.

Use the map above and below to see how concepts connect, and the menu (top right) to switch theme or language.