Play with an idea until it makes sense, then practise it until it sticks. Two ways in — pick whichever one you came for.
Dozens of things you can actually turn a dial on. Fire electrons one at a time and watch interference build. Push R₀ past one and watch an outbreak take off. Every number on the page is computed, not illustrated.
The Class 6 and Class 9 syllabus, chapter by chapter, with worked examples and a question bank to test yourself against. Every question carries a written explanation, not just a right answer.