How does an arch work? Lego MOC

First of all, we need vertical walls on the left and right, scaffolding with adjustable height and bunch of stones used for the arch. Note the shape of the stones and especially the one to the right. It is called keystone and it is going to be placed on the top of the arch.

So now we can move the scaffolding under the future arch and start building.

The arch is now finished as we put the keystone on place as the last one.

Now we can lower the scaffolding and see how the arch sits down and keeps it's shape.

And why were the walls on the left and right so important? They hold the whole weight of the arch. Here you can see how the arch collapsed when we removed the left one.

I hope you enjoy this small educational MOC :)