This model is made by 3D printing and it has four layers, a layer on the bottom and a couple of layers that come up to the top and then finally a circle in the very center. I'm going to show you how we can put this together and some things that students can learn from it. So the base is made with 12 pieces around the circumference. I put a rubber band on it just to hold it together and it provides a spring line for the rest of the shell. I put an elastic string through the next row just to make it a little bit easier for me to build it. There we go. There's the next layer. One of the things you'll notice is that the design is such that if you put a single block on here will fall off because the center of gravity is too far towards the inside. But the other thing you can see is that if you lean the one block against the neighboring block that it will stay in place. When you build an igloo that's exactly what you do. You build this actually not as a whole line but you build it as a helix from the bottom to the top and so you can always introduce the latest block by leaning it against the neighboring block. So if I put in one more block here that completes that ring. The next ring is trickier to build because it's leaning inwards more strongly and the easiest way to do that is to build one side, put the center in and then build the other side. And then finally I'll pop in these last couple of pieces. And there we have our igloo. The interesting thing is you can move that center piece and there's no danger of it collapsing. Now there are a whole bunch of other things that we can demonstrate with this model. In this version, this is the very same design. I have put elastic strings along in the meridional direction in order to hold these pieces together. And what you can see is some of the following things. So I've got a rubber band around the bottom and I've got strings in the meridional direction just to hold the bricks together. Suppose that this was a masonry dome and you put a large cupola or some other kind of a load at the center. What could happen is this. Notice how the space is open up between the blocks. If this was made a masonry, you can imagine that this would fall down as a result of that loading. What they do in many real domes is to put steel bars or steel reinforcing in the hoop direction so that this cannot happen. If I remove the rubber band from the bottom, so if I loaded on the table, you can see that what would happen is that the individual stones or blocks in the dome would separate and the dome would fall down. If I remove the rubber band from the bottom, then I no longer have a springing line here. And if I apply load to this, it just opens up flat. So you can see the importance of a springing line here because then it redirects the forces upwards and downwards and causes this to be a stable structure. The rubber band in the notch simply provides a way to generate that required springline. The other thing I can do with this model is I can put this half in the holder. I can put this on top and then it demonstrates a spherical pressure vessel.