learning a skill isnt really scalable,u still have to go out and do the work and earn the income,and ur earning power is limited by how many hours u can work a day,its not like investing in real estate where u can buy a hdb and shoebox condo and rent them out and buy another condo,giving you 3 income streams.or buying blue chip dividend companies,u buy 10,000 singtel shares u get $2000 dividends,the next year u buy another 10000 shares and now u get $4000 dividends and so on and so forth until 10 years later u are getting $29732 dividends a year assuming u reinvest the dividends.ur earning power is unlimited assuming u keep buying 10,000 shares every year,until one day 40 years later u are looking at $5,000,000 worth of singtel stock earning u $250,000 dividends a year.
another way is to start a business and grow it until u have 50 or 100 people working for you but thats a more complicated business.