SB 4.22.18

sanat-kumāra uvāca
sādhu pṛṣṭaṁ mahārāja
 sarva-bhūta-hitātmanā
bhavatā viduṣā cāpi
 sādhūnāṁ matir īdṛśī
Word for word: 
sanat-kumāraḥ uvāca — Sanat-kumāra said; sādhu — saintly; pṛṣṭam — question; mahārāja — my dear King; sarva-bhūta — all living entities; hita-ātmanā — by one who desires good for all; bhavatā — by you; viduṣā — well learned; ca — and; api — although; sādhūnām — of the saintly persons; matiḥ — intelligence; īdṛśī — like this.
Translation: 
Sanat-kumāra said: My dear King Pṛthu, I am very nicely questioned by you. Such questions are beneficial for all living entities, especially because they are raised by you, who are always thinking of the good of others. Although you know everything, you ask such questions because that is the behavior of saintly persons. Such intelligence is befitting your position.
Purport: 

Mahārāja Pṛthu was well conversant in transcendental science, yet he presented himself before the Kumāras as one ignorant of it. The idea is that even if a person is very exalted and knows everything, before his superior he should present questions. For instance, although Arjuna knew all the transcendental science, he questioned Kṛṣṇa as if he did not know. Similarly, Pṛthu Mahārāja knew everything, but he presented himself before the Kumāras as if he did not know anything. The idea is that questions by exalted persons put before the Supreme Personality of Godhead or His devotees are meant for the benefit of the general people. Therefore sometimes great personalities put themselves in that position and inquire from a higher authority because they are always thinking of the benefit of others.