Let us look up and see whether the full moon has risen."

At that very moment the full moon did appear in the sky. Abhiramibhatta was steeped in truthfulness. By mistake he had spoken an untruth but Amba made it the truth by hurling her ear stud into the sky causing it to shine like the full moon. The blessings as well as the curses of great men come true because of the force of their innate and habitual truthfulness. This is the "incidental benefit" they derive from their habit of truthfulness. But truthfulness must not be practised with the deliberate intention that what one speaks must come true. Power such as this is earned unintentionally and unconsciously. 

A man will purify himself completely if he performs the forty samskaras and adheres to principles like non-violence, truthfulness, non-covetousness, cleanliness and also controls his senses. He will then develop the maturity and wisdom to find out who in truth he is, who Isvara is and what the Ultimate Reality is.

Source: "Hindu Dharma"- English translation of "Deivathin Kural", a collection of invaluable and engrossing speeches of Sri Sri Sri Chandrasekharendra Saraswathi MahaSwamiji.