More than 230 of the world's wealthiest people, including Elon Musk, Bill Gates and Warren Buffett, have promised to give at least half of their fortunes to charity within their lifetimes or in their wills by signing the Giving Pledge. Industrial titans like oil baron John D. Rockefeller, automotive entrepreneur Henry Ford and steel magnate Andrew Carnegie established massive foundations that to this day have big pots of money at their disposal despite decades of charitable grantmaking. This kind of control over funds after death is usually illegal because of a you-can't-take-it-with-you legal doctrine that originated 500 years ago in England.