Lol'd

@scarface
oh that one.
I'll just state my theories,
What I think is that because it's pure water the surface tension and other things make it hard to bubble(as they say boil), all this preasure inside the water wants to escape but can't because it's got no impurities that allow a gap in the surface tension. it explodes because putting a sugar cube in breaks the surface tension.

But that's just my theory, I don't know whether it's true.

EDIT: oh and the microwave heats from the inside out. This means the hotter water is trapped on the inside. So when you drop a sugar cube in, it will allow the hotter super heated water out, it explodes.

Actually, the OP has merit, it was proven some time ago that pure water with absolutly nothing added, cannot generate enough friction to be heated by certain means (like microwave)

This does NOT mean that it cannot be boiled and the water has to be literally pure, not just "distilled".

I'm talking just Hydrogen and Oxygen, nothing else.