Some materials have a shape memory effect, meaning they return to their original shape when heated.

So for anyone genuinely curious, the material shown in the video is Nitinol, an alloy of nickel and titanium which is really interesting because it exhibits both shape memory and super elasticity properties. It's an expensive material to buy and also difficult/expensive to manufacture, so it's considered specialized and really only widely used in industries like medical devices or aerospace. The shape can be modified to virtually anything, so long as you can make the material into that shape in the first place. You heat treat the Nitinol at high temperatures (typically 500C+) in your desired shape to "lock it in". Additionally, the temperature at which Nitinol has shape memory (springs back to its original shape) is adjustable for your application. Random fun fact: A guy tried to make soup bowls out of nitinol. They can be stored flat and when the hot soup is poured in, the ends bend upwards to create the bowl. The original design failed largely do to cost. Source: I am a Biomedical Engineer who's job it is to design and manufacture medical devices out of nitinol.

Nickel titanium, also known as Nitinol. #DYK Nitinol alloys exhibit two closely related and unique properties: shape memory effect and super elasticity.

Artist Daniel Wurtzel is best known for his kinetic sculptures and installations using air and lightweight materials that fly. This is his air fountain at Centro Cultural Banco do Brasil in Rio de Janeiro

Inner turmoil is also like this This tree was stuck by lightning, the bolt passed through it, heated up the sappy interior, which is a better conductor of electricity

Shape memory alloys (SMAs) are a class of smart materials that can retain their previous shape when subjected to external stimuli. This includes thermomechanical or magnetic gradients, as well as temperature, stress, moisture, electric or magnetic field, pH, light, or chemical compound SMAs can undergo a seemingly permanent deformation below a critical temperature and then recover their original shape upon heating. One of the first popular alloys was Nickel-Titanium, commonly known as Nitinol. SMAs are popular for a wide range of applications. For example, Ni-Ti wire is used in orthodontic applications to hold teeth tight with a constant stress. It resists permanent deformation even if it is bent. SMAS (Ni-Ti) are also used to make eye glass frames and medical tools.

This gigantic shape was caved out from a stone 1200 years ago

Reminds me of this:

They're shape memory alloys. Before anyone starts doing this to things at home, it doesn't work with anything except shape memory alloys. A paperclip isn't a shape memory alloy, it's steel. So don't be disappointed when it doesn't form its shape back when heated up.

But, how would the original shape be made in the first place????

How fascinating is this