If you hadn't heard of Opendoor Technologies (OPEN -2.81%) before June, chances are you've heard of it since. Opendoor stock has had an unparalleled rally over the past few months, fueled by retail investor power and the strength of social media. Opendoor stock gained nearly 2,000% from its low in June through its year-to-date high in September, but it's been starting to fall since then. Carvana stock had a similar rally after plunging last year, and the company has made a serious turnaround. If you buy today because you believe in the company's future, and you have a strong appetite for risk, now may be a great time to buy shares.