I find the idea of changing materials part-way through a piece interesting, you might not be able to fuse them but I'm imagining something similar to a dovetail joint, printed in place and utterly permanent. Essentially just mechanically interlocking the different materials during printing.

That's exactly what the spiral arms are, they can't be the same stars orbiting together in that shape as that would imply a rigid body rotation. The situation where everything moves around together as if it were nailed to a rigid cosmic disc doesn't work because the orbit time of the stars at the centre of the galaxy is less than that of the stars at the edge. This is a consequence of the orbital physics, it's essentially the only way the forces can balance.

So, the stars in the centre whiz around quickly (in cosmological time anyway) whilst the ones at the edge take forever. The spirals are simply areas of higher star density but they are not the same stars all the time. This region does rotate but more slowly than the stars contained within it. So, why are there areas of increased star density? No-one's entirely sure but it seems likely that these are actually regions with higher rates of star formation, with many young, short-lived blue stars.

80 kg of water is 0.08 m^3 (1 litre of water being a cube 0.1 meters each side and of mass 1kg), if you turn it into steam it expands by about 1700 so assuming atmospheric pressure gives 0.08*1700= 136 m^3 of steam as the OP stated.

This is well known to be possible, has been done for years, and you can buy commercial test equipment that sends spoof GPS signals (for testing GPS receivers obviously). More importantly there's another simpler way that cannot be dealt with by signing - just relay GPS signals from elsewhere.

If you capture GPS data at a point in space and retransmit the whole lot with enough power that the receiver sees only your signals then the receiver sees all the same phase relationships that put it in the location where you captured the data. It has no way of knowing it's been delayed by a few microseconds and signed GPS signals would look perfect. The only way around that is to compare it with inertial navigation, loran, etc or perhaps to have a very accurate clock on board to try and spot the extra delay.

You can make a gun-type bomb with impure plutonium, what you can't do is make one short enough to deliver in a missile or a plane. Built diagonally on the 100th floor of an office building or more feasibly at ground level in a dockside warehouse however....

You "just" need to increase the assembly velocity, and there are ways of doing that which are simpler than building an implosion device. And as you point out a fizzle is still a significant yield, and much dirtier.

Give a final year physics student a mechanical workshop and the plutonium, all they'd need is the funding.

Unlikely to change for that reason. Cheap beers might come in standard bottles but premium beers tend to have custom bottles anyway with brand names and logos in the glass. As long as they see a benefit in branded bottles they can have them made any size they like at no change in the cost.

I assume you're Australian then? In the UK canned beer is 440 ml or 500 ml. Only soft drinks tend to come in 330 ml cans. 250 ml bottles are very rare here but 330 ml is common for many imported beers.