sand grains accumulate to make a tower a tower (idiom); by gathering small amounts one gets a huge quantity; many a mickle makes a muckle
聚：集合，聚集。把细沙堆积成宝塔。比喻积少成多。
主谓式；作宾语、定语、分句；指积少成多
聚细沙成宝塔。原指儿童堆塔游戏。后比喻积少成多。
《妙法莲华经·方便品》：“乃至童子戏，聚沙为佛塔。”
