How does Ymir turn back into her human form? Late in the manga, we know that titans are actually transformed humans, and that not all people who transform into titans can turn back into human form.

I did a lot of research on Ymir and there are many theories regarding how she turns back to her human form for the first time. How does it work?
The answer contains heavy spoilers to anime watchers. You have been warned!

It's been heavily hinted (almost confirmed, really) that titans are created by transforming humans in some way. Ymir was transformed to a mindless titan over 60 years ago, and was unable to return to her human form for a very long time.

We already know that several powers (such as titan shifter powers, titan control powers, and "The Memories") can be transferred by eating. This conclusion was reached by Hanji in chapter 57.

When Ymir ate Reiner's and Berthold's friend (who was also a titan shifter), she gained his titan shifting abilities, and was able to return to human form.