A free mathematical playground with interactive courses, puzzles, and activities covering everything from symmetry to fractals. Maths you can actually poke at.
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The courses ask you to explore and answer questions as you go, rather than handing you a wall of formulas and hoping for the best.
Fractals, graph theory, origami, and the history of mathematics offer several ways in, including for people who stopped enjoying maths somewhere around fractions.
Mathigon is free for students, teachers, and parents. Most content works without an account; signing in lets you save progress.