Something Interesting in the Woodshed

These are a bit like the chairs we found in the woodshed. You can see why we chopped them up.

'What are those over there?' says Dexter, pointing at two spindly old chairs in the corner. They are covered in newspaper.
I shrug.
'Dad maybe. They're probably going to the dump...I know! We can chop them up for firewood!' I say.
So we start chopping, one chair each. It's great fun and by the time we are finished, I am quite warm. The chairs are gone and in their place is a satisfying pile of wood for the fire.
'They'll be really pleased,' I say, as we begin lugging it into the kitchen.
'Hello, Wilfred,' says Dad as he fills the kettle. 'Hello, Dexter-what have you two boys been up to?'
Then he drops the kettle in the sink and turns as red as Father Christmas' coat.
'Where did you get that wood?' he asks very, very quietly.
'Your shed,' says Dexter, 'they chopped up really easily.'
'Shed...they...chops,' repeats Dad and his eyes are all poppy.
Something is wrong, I can tell. 'I was just doing what you asked me to,' I say, all the same I'm glad I haven't brought the axe in as well. Dad looks as though he's about to explode. Now what?