Elon Musk promised to go eat a McDonald's children's Happy Meal on TV if the fast food chain accepted his dare. (Credit: Frederic J. BROWN / AFP)It all started on Twitter. Elon Musk, the high-flying CEO of Tesla Motors, reached out to McDonald's, daring it to accept Dogecoin, the cryptocurrency featuring the head of a shiba inu dog. The entrepreneur, whose projects can be both visionary and fantastical, promised to go eat a McDonald's children's Happy Meal on TV if the fast food chain accepted his dare. Elon Musk posted a series of tweets, often humorous, referring to the steep tumble in cryptocurrency markets. Other Twitter users called on Bitcoin holders to find work at McDonald's to make up for their lost value. McDonald's Twitter account was quick to respond, beating Elon Musk at his own game. The chain proposed that Musk's company Tesla accept the "Grimacecoin," a cryptocurrency straight out of the imagination of McDonald's communication team. Grimace is an imaginary purple character found on the packaging of some McDonald's food in the United States.