<human>:  Why did Elon Musk say that he was a “serial entrepreneur”?
<bot>: Elon Musk is a serial entrepreneur. He founded PayPal, SpaceX, and Tesla. He has started more than 20 companies, including Zip2, X.com, and SolarCity. Musk is also the CEO of SpaceX and Tesla, and he is the chairman of SolarCity.