Summer Anthem. Stocks started out strong Thursday and just kept getting stronger, as investors cheered upcoming talks between the U.S. and China. •...and explain why Chipotle Mexican Grill (CMG) was the S&P 500's worst-performing stock. Forget Your Troubles and Just Be Happy  Stocks shook off Wednesday's blues to close broadly higher, as optimism about China pushed Turkey to the back burner, although there was good news on that front, too.