(The man agreed and whilst the bird was in his hand he said), “Do not lament what has been lost.” The man let it go and it perched on the tree and said, “Do not believe something that can never be.” Then when it reached the mountain and it proclaimed, “O unfortunate one, if you had killed me then you would have found in my belly two pearls, each one worth twenty mithqals.”
The man bit his lip out of grief then said, “What was the third?” It replied, “Did you forget the first two? So how could I tell you the third? Did I not tell you not to lament what has been lost and not to believe something that can never be? I am flesh, blood and feathers; there is no twenty pieces in me! So how could there be two pieces weighting twenty mithqals in my belly!” Then it spread its wings and flew away.
This story can be found in many works of Imam Ghazali (may Allah be pleased with him) and others, this is from Nafhat Al-Arab/Breeze of the Arabs p.58.
One mithqal is about 3 grams in weight so the bird tricked the man saying he had sixty grams worth of pearls in its belly. Smart bird!!