This is one of the nostalgia toys in Hong Kong. It was a popular toy in the past because it was made with plastic and cheap enough for children to afford one. Since its pattern is similar to water melon with red white stripe, people call it "water melon ball".
 
西瓜波又稱紅白波，是香港從數十年前流行的一種球類玩具，是一種塑膠製成品，有紅白兩種顏色相間。當年的小孩因為缺乏玩具，而西瓜波的製作成本很低，價錢十分便宜，因此十分流行。