That's a good answer. The only reasonable way to double your money is to buy something that you can resell in the short term for a profit of 20% or more. Do that two-three times and your money is doubled.
Alternatively, invest in something where you can act to increase the value in the short term and recoup your investment in a few months. Similar to buy and sell but you only invest in part of an existing business.
Examples.
Buying web businesses that are sad because the builder was cheap and hired an incompetent. You buy it cheap, fix all the problems and sell a shiny new bulletproof site to a promoter/marketing person who knows that they can use it to earn ongoing revenue. Idea guy botched the job by outsourcing, you the tech guy fix the tech problems (and maybe some design UI UX stuff) then sell to the sales/marketing business guy who has a long term plan.
Another example. A waitress quits her job and invests her savings at a grimy sad restaurant across town. She also takes a waitressing job at minimum wage there. Every night after closing time she cleans and decorates. And working in the daytime she wears flirty clothes, keeps up her best smile and cheerful manner. Word gets out and new customers come to see what's happening. Business doubles and triples. After 4 months, the owner pays her back her investment plus 40% and gives her a regular job as manager/waitress at a normal wage. She works there for another year, keeps building the business and hires the right replacements. Then on the basis of this transformation she gets a job as catering manager at a small regional motel chain and gets into renewing their restaurants. With the money she saves up, she quits after two years and buys a small restaurant. After a year she sells it for double what she paid and goes on to the next. People actually do this kind of thing. It works, but it requires guts, intelligence and hard work.