Build something on sand:
To put something in an unstable position by failing to give it a secure foundation.
用 “在沙子上建造某物” 来比喻 “事物基础薄弱或没有基础”。虽然它可以描述 “建筑物等有形的事物根基不稳固”，但往往多用来指 “论点或想法毫无根据”，多用于被动语态中，即 “be built on sand”。
例句
Without any planning or financial backing, Derek is building his company on sand.
没有任何计划和资金支持，德里克正在创建的公司基础不牢固。
Her argument made no sense and was built on sand.
她的论点说不通，而且毫无根据。
At first I thought Sarah’s plan for the project was built on sand, but then she presented the research she’d done.
起初，我认为莎拉的计划毫无根据、不切实际，但后来她展示了她为此计划先前做过的研究调查。
