How do you convert a tourist visa to a work visa in US?

While in the US. if one is traveling to the US. primarily for tourism but they want to take a short course of study that is recreational and not for credit towards a degree, and the course is less than 18 hours per week, then this is allowed on a B2 visa. For example if one is taking a vacation to the US, and during that time they wish to take a two-day cooking class for their enjoyment, and no credit is earned, then this would be permitted on a B2 visa. But when you are in the US on a tourist visa you can not work.