The history of the katana goes as far back as 11th century Japan, and the sword variety has endured because it can be easily used with two hands, and of course, because of its strong, long blade. They’re still made today, and the process is fascinating. You can watch the entire process in the video below, but we’re going to focus on how the edge is made to be so damn sharp.

The featured video at the top of this post (via Sploid) shows a blazing hot blade being quickly dipped in a vat of water, at which point it bends downward before recoiling and concluding its transformation in the familiar katana shape, with a slight upward curve. The question is, why does it happen this way?

It has to do with the shape of the blade, which features one side (the edge) that is thinner than the other (the back). In the video, the edge is face down, and because this side has less material, it cools down and contracts more quickly than the back, which results in the downward curvature. Once the back has a chance to cool down as well, the blade regains its normal shape. This process results in a stronger macro-structure of the metal, and therefore a stronger sword.

Many people might now realize how intricate the sword-making process really is, so for further viewing (and if you have an hour to kill), check out the 50-minute documentary on the history of the katana below.

Featured image: Rama