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In Shingeki no Kyojin, Armin's grandfather dies when he is sent to recover Wall Maria. And in the episode when Eren gets eaten, the face and the look of the giant who eats him is exactly the same as Armin's grandfather.
That is strange. While their victims are not digested by the titans they are chewed, so I wouldn't think that this is Armin's grandfather somehow transformed. Maybe that's how the humans are "sampled".
@Washu: I believe that humans can be turned into (mindless) titans by some means. The obvious example being Connie's mother, even though it wasn't explicitly stated.
I personally think that humans can be transformed into titans. I came up with this theory because a titan doesn't need nourishment and it seems all they need to live is sunlight. The idea hit me when I watched an episode where someone stated that because a titan didn't have a digestive system, they simply regurgitates them (it's really scary) and the victims are all covered in this weird looking goo. So maybe the titans do this for reproduction. I still don't know what caused this, but it's just a theory.
Also when I read about the incident in Ragako, Connie's hometown, the buildings were destroyed, but there's no blood or bodies, the horses are still in the stable, and they found a titan that looks like Connie's mom.
Well, that's my theory and my personal opinion. I just discovered this anime and started watching it 3 days ago.
It's not just conjecture at this point. Besides the titan looking like Connie's mother, it was lying on Connie's house, but clearly unable to have gotten there on its own due to its limbs being too small to carry it. The titan also seemed to say, "Okaeri" (welcome home) when it saw Connie. As to how exactly people get turned (permanently?) into titans against their will, this is not explained yet.
Humans can turn into Titans after injury. Due to the fact that the Humans are chewed. This leads for the potential of a transformation to occur before he dies.
In this scenario the above is an explanation for why it could be him, but since it's not directly commented on in the show there is also the potential for a reuse an image they already had.
spoiler: When the main character bleeds he turns into a titan. If this was due to some dormant gene that others have but can't control, then in their near fatal chewed status they might turn into a mindless titan permanently.
Titans are humans. In the next season there is a former Titan (Ymir) with the same powers (albeit even weaker) as Eren that was created after she ate another person that had been experimented on by Dr. Jaeger and as a result regained her most of her sanity.
There's a one-off OVA episode (Ilse's Journal) where Hanji tries to capture a Titan that corners Ilse and bows to her believing she is Ymir, the Cured One.
To update this with the release of recent manga chapters, in Chapters 87-89,
it was shown where the mindless titans in Paradis Island came from. They were Eldian rebels caught by the Marleyan government. Through an injection, they are turned to mindless titans. Why do the giants in Shingeki no Kyojin start to look like the characters that got eaten? That's purely coincidence and that is not how it was in the manga.