What are the best ways to earn money while traveling around the world?

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If you have some money, international trade. At every country you visit, find something that you can ship back to your home country and sell for profit (of course something legal). Like when I visited Mexico last time, I found some cheap but good "Hecho en Mexico" tires because I got a flat. If I had more time, I would try to locate the manufacture and see if I can get a truckload shipped back to the US tax free via NAFTA agreement.