The Four Tiers
Toddler · Ages 3–5
Pencil control, letters, numbers and shapes. Big line art, one idea per page, and something to colour at the end of every book.
Child · Ages 5–9
Real explanations with real numbers. The solar system drawn to true scale, simple machines built from a ruler and a length of string.
Teen · Ages 10–14
Algebra, chemistry and coding logic. Worked examples before every practice set, full answer keys at the back.
Advanced · Ages 15+
Calculus, organic chemistry and Python. Senior high school and first-year university, derived rather than memorised.