Surah 21. Al-Anbiya, Ayah 57

He wants to convince them of the powerlessness of their idols. But he does not do it underhand. He tells them that he is going to do something when once they are gone and their backs are turned to the idols,-as much as to say that the idols are dependent on their care and attention. Apparently the people are amused and want to see what he does. So they leave him to his own devices.