山羌師傅在做美濃紙傘。從上面看，紙傘是一個圓，有 24 根傘骨，每兩根之間是 15°。孩子從最上面那根傘骨開始，拖著傘面順時針轉開，做出一片扇形。第 1 關傘邊有像量角器的刻度，要轉出指定的圓心角；第 2 關沒有刻度，要做出「幾分之幾圓」（例如 1/3 圓 = 120°），或看一片扇形寫出它是幾分之幾圓；第 3 關照著大傘做一片圓心角一樣的小傘面（圓心角和半徑長短無關），以及把兩片扇形合起來（1/4 圓 + 1/6 圓 = 90° + 60° = 150°）。
適合年級：五年級（約 10～11 歲）。對應 108 課綱（十二年國教）數學單元：五年級「扇形和圓心角」。各版本教科書的單元順序不同，完整對照請看國小數學課綱對照表。免費、無廣告、不用註冊，手機、平板、電腦打開就能玩，題目有注音，也會念出來。
陪玩小提醒：一整圈是 360°，所以 1/4 圓是 90°、1/3 圓是 120°。可以問問孩子：大披薩和小披薩各切一片四分之一，哪一片的角比較大？
Master Muntjac makes Meinong paper umbrellas. Seen from above an umbrella is a circle with 24 ribs, 15° apart. Children drag a panel open clockwise from the top rib to make a sector. Level 1 has protractor numbers round the edge and asks for a given central angle; level 2 has none and asks for a fraction of a circle (1/3 of a circle = 120°) or for the fraction a given sector makes; level 3 asks for a panel on a small umbrella with the same central angle as one on a big umbrella (the angle doesn’t depend on the radius), and for one panel equal to two sectors put together (1/4 + 1/6 of a circle = 90° + 60° = 150°).
Suits Grade 5 (about age 10–11) in Taiwan. Taiwan curriculum (12-year basic education) unit: Sectors and central angles (Grade 5). Free with no ads and no sign-up, in Traditional Chinese with zhuyin and in English, read aloud. See the Taiwan elementary maths curriculum map.