The shares of two meme stocks surged in Thursday trading, adding an unexpected twist to a stock market that’s been falling steadily for the past month.

GameStopIt jumped by more than 30 percent and was repeatedly halted due to volatility. Stock of theater chain AMC EntertainmentMore than 20% of them were popped.

GameStop was and AMC were big news when a bunch of retail investors coordinated trades through online chatrooms. This enabled them to cause massive short-sizing in the stocks, which are widely loathed by other hedge funds. These speculative stocks saw huge rallies that caused great pain for hedge funds as well other short sellers.

Stocks have since retreated from peak levels and short-sellers are building positions again. FactSet reports that AMC’s short interest is 19.5% and GameStop at 21.4%, according to FactSet.

Hedge funds may make large bets on a company, sometimes resulting in dramatic stock moves. This is also known as a short squeeze.

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