Now we know why
One of the only sure-fire ways to kill them was by slicing an area one meter long and 10 centimeters wide on their nape. This was the location of the central nervous system of the Titan. In Pure Titans, it was only a spine of the human that was absorbed but in the Nine Titans, it was an entire human body.
No, the regular titans, not the titan shifters, were humans who were injected in the back of their nape with some sort of liquid, most presumably titan spine fluid, by the nation of Marley.
In the tribute game there are currently only 3 weak spots on the titans (not including the crawler and Colossal types which only have one). These include the eyes, the nape of the neck and the tendons.
The nape of the neck is the key weakness of every Titan whether they are a Pure Titan or a Titan Shifter. Titan Shifters in the anime have done their best to protect their necks from enemy attacks by using their hands to cover up the vital spot. But, considering how important it is in battle, more could be done.
Pieck's Cart Titan is arguably the weakest of the Nine, but it comes with several advantages. Unlike its eight brethren, the Cart Titan walks on all four legs, making it an excellent method of transport. Pieck often has large structures attached to her back, which carry both people and weapons.
Because it is impossible for anyone to surpass the Founder, each person who gained the power of the Titans was fated with the "Curse of Ymir" (ユミルの呪い Yumiru no Noroi?), which limited their remaining lifespan to only 13 years after first acquiring it.
This makes sense as the nape of the neck is where the brain roots connect with the spinal columns, and once this connection is severed, the Titan loses contact with the rest of its body, losing its regenerative abilities in the process.
It will eventually be exlpained in the anime, but if you want to know: Non-shifters are normal humans that were transformed in mindless titans via a serum. Their spinal cord is still in the nape, so that is why the nape is the weak point. Thats also why a non-shifter can return to human form if they eat a shifter.
1 Eren Yeager
The protagonist of the story, that resorted to mass genocide to protect the things he loves, is at the top of the list. Before he decided to do that, he had been killing titans as a titan shifter of the Attack Titan.
This Titan was the first Pure Titan shown to speak words with meaning. The second was Ms. Springer. This Titan was the only common Titan seen that displayed a range of emotions.
Eldians who possess the Power of the Titans can revert to their human form and retain abilities such as speech and human emotion while in Titan form.
Eren Yeager
The War Hammer Titan is the final Titan to make its presence known in the series, but its impression is still a really strong one. Its mastery of crystallization means that the War Hammer Titan can easily destroy its enemies with the crafty use of its powers.
Rod Reiss' Pure Titan
Considered as one of the most gruesome-looking Titans in Attack on Titan, Rod Reiss' Titan form is very ugly with a huge torso and disproportionate, very thin limbs and head. This Titan drags itself on its belly as it slitters its enormous form around.
The Wall Titans are similar in appearance to the Colossal Titan in that both their surfaces are covered primarily in muscle tissue and lack skin (although the Colossal has exposed bone over the muscle tissue of its form).
At the top of these special Titans is the Founding Titan, who had been in the Royal Family of Eldia.
The Nape Of The Neck
In fact, women ranked the nape of the neck above the breasts and nipples as an erogenous zone, according to a study published in the journal Cortex. "It's such a sexy place because it's such a vulnerable place," says McCombs.
1 Mikasa Killed Eren, Destroying The Founding Titan For Good
Mikasa entered the mouth of the Founding Titan where Eren's body was sheltered and briskly decapitated him. Eren's death stopped the power of the Titans for good, which meant Titans could no longer terrorize the world.
The pterion is a craniometric point at the point where the greater wing of the sphenoid bone, the parietal bone and the frontal bone meet. It is the weakest point of the skull.
It's mostly just a design choice by Isayama. He thinks they look more unnerving that way (and he's not wrong, the smiling ones are usually much creepier). That's not really a smile. Most of the titans have disproportionately small lips in comparison to their jaws, so their teeth are bared.
In the most recent chapter Eren said [wrote] that he couldn't talk in Titan form because of the shape of his mouth.
Put simply, Titans eat people in the hopes of regaining their humanity, and if they consume the spinal fluid of a Titan Shifter - one of nine people who can transform into Titans at will - they will return to normal.
Oceanus: Titan God Of The Sea & Water
The eldest of the Titans, Oceanus was married to his sister Thetis.
When he is capable of explaining what's going on while piloting a Titan from the nape of the neck, he says he can't really feel the injuries. Titans heal incredibly quickly, recovering from severed limbs and gouged eyes within minutes.
Eren Yeager - Wikipedia
that he first met Grisha about 20 years ago. At this point the attack on Wall Maria which coincided with Grisha's death happened 5 years ago. So Grisha had in fact been inside the Walls for 20-5 = 15 years!