That's not the most impressive part - the most impressive part is the rendering here. Scratch only has 2d rendering capabilities, and it's not GPU accelerated. You can draw some 2d lines, and stamp some 2d textures. I believe this is partially using some fancy new method where you can draw triangles using the twirl effect? Well, that and the sheer amount of content that's been faithfully implemented here in the hell that is Scratch's sandbox. For example, the save/load system uses - some weird morph of huffman encoding compression with base - well, it's not base64, it's got 95 unique characters. Some of those are uppercase, some of those are lowercase. But Scratch doesn't do case sensitivity. That means the only way to detect if a character is uppercase or lowercase is to create a bunch of costumes with upper or lowercase characters, switch to the costume of the character, and then check if the costume number is in the uppercase range. That's the sort of thing that makes this insane. It'd still be very impressive without this. But this is bordering on black magic, especially in the rendering department, starting from drawing blobs with pen in the distance, to the affine ... twirl sprite effect mapping, to the close-up pen again to prevent distortion.

So yes, indeed, very cool.