Pick a number of sides. Three gives you a triangle, four a square, and as the count climbs the shape edges toward a circle — the same idea from the homepage, now in your hands.
A regular polygon has sides of equal length and angles of equal measure — a triangle where all three sides match, a square, a regular pentagon, and so on. As the number of sides n grows, a regular polygon starts to look more and more like a circle, because each side gets shorter and the shape has less room to bend.
In the Perimeter & Area tab you can choose n and watch both values recompute live. In the Prism tab, that same flat shape gets stretched into a solid, and you'll see how Lateral Surface Area, Total Surface Area and Volume all build directly out of the 2D formulas.