How to acquire knowledge? - Sanskrit sloka

 गुरु शुश्रूषया विद्या पुष्कलेन् धनेन वा। अथ वा विद्यया विद्या चतुर्थो न उपलभ्यते॥ 

Knowledge is acquired by serving the master or by giving enough money or in exchange of knowledge. A fourth way is not seen.