SB 7.5.55

atha tāñ ślakṣṇayā vācā
 pratyāhūya mahā-budhaḥ
uvāca vidvāṁs tan-niṣṭhāṁ
 kṛpayā prahasann iva
Word for word: 
atha — then; tān — the class friends; ślakṣṇayā — with very pleasing; vācā — speech; pratyāhūya — addressing; mahā-budhaḥ — Prahlāda Mahārāja, who was highly learned and advanced in spiritual consciousness (mahā means “great,” and budha means “learned”); uvāca — said; vidvān — very learned; tat-niṣṭhām — the path of God realization; kṛpayā — being merciful; prahasan — smiling; iva — like.
Translation: 
Prahlāda Mahārāja, who was truly the supreme learned person, then addressed his class friends in very sweet language. Smiling, he began to teach them about the uselessness of the materialistic way of life. Being very kind to them, he instructed them as follows.
Purport: 

Prahlāda Mahārāja’s smiling is very significant. The other students were very much advanced in enjoying materialistic life through religion, economic development and sense gratification, but Prahlāda Mahārāja laughed at them, knowing that this was not actual happiness, for real happiness is advancement in Kṛṣṇa consciousness. The duty of those who follow in the footsteps of Prahlāda Mahārāja is to teach the entire world how to be Kṛṣṇa conscious and thus be really happy. Materialistic persons take to so-called religion to get some blessings so that they can improve their economic position and enjoy the material world through sense gratification. But devotees like Prahlāda Mahārāja laugh at how foolish they are to be busy in a temporary life without knowledge of the soul’s transmigration from one body to another. Materialistic persons are engaged in striving for temporary benefits, whereas persons advanced in spiritual knowledge, such as Prahlāda Mahārāja, are not interested in the materialistic way of life. Instead, they want to be elevated to an eternal life of knowledge and bliss. Therefore, as Kṛṣṇa is always compassionate to the fallen souls, His servants, the devotees of Lord Kṛṣṇa, are also interested in educating the entire populace in Kṛṣṇa consciousness. The mistake of materialistic life is understood by devotees, and therefore they smile upon it, considering it insignificant. Out of compassion, however, such devotees preach the gospel of Bhagavad-gītā all over the world.