Why does raw onion catch fire in the microwave?

Question

I microwave raw onion and it's filled with smoke within few seconds.

Answer 1

If you chop it and add it to a microwavable lidded bowl [ not tightly lidded, just rested on the bowl ], with a little butter and water its  usually fine.

Answer 2

Lots of electrolytes in onions, that's why they bring people to tears.

Answer 3

Nothing better to do today, eh?