Why would raspberries spark in a microwave?

Dave - I imagine it's related to microwaving grapes. If you get a grape, cut it almost in half, so it's got a little bit of skin holding the two halves together and then open it up, and put it in the centre of the microwave where there's lots and lots of power. a microwave oven cook things by forcing electric currents to flow backwards and forwards in them, and you end up with a lot of current flowing through this little bit of skin that then dries out. Then you get the current wanting to flow across the gap and it actually jumps as a spark. So my guess, what is happening with your raspberries is you're getting raspberries very close together or just touching - that gap then dries out and it sparks across the gap.

Chris - Also, raspberries are lots of little mini segments aren't they and I wonder if maybe each of the mini segments are behaving a bit like a half of a grape with the segment next door, do you think?

Dave - It's possible although I would've thought that the contact between them was fairly good especially as it's heating up and getting mushy.