Maybe I should admit that I'm utterly hopeless at riddles - I just thought conundrum was too good a word to keep to myself. I can't play the violin either, but I love to listen to those who can.

I got the impression that one or two didn't get the whole thing about the COD. If you did, skip this bit, if not ...
co is plural, as in "Stalky & Co"
od[d] is "singular", i.e. "strange". I keep hearing that Americans aren't really into puns, so maybe that's not obvious.

Here's a joke about a riddle:
Col. Cholmondley-Featherstonehaugh (pronounced Chumley-Fanshaw; don't ask!), RHA (retd.), spent some time in Singers (that's Singapore to you and me) after he left the army. He went into the bar at Raffles' Hotel, where he was staying, and got chatting to the barman. The conversation turned to riddles, and the colonel said, 'I've always loved riddles, but I can never work out the answers.'
'I've got a good one,' said the barman, 'would you like me to tell it you?'.
'Go on then, I'm game!'.
'"It isn't my sister, it isn't my brother, but still it's the child of my father and mother". Any ideas?'

Well, the colonel thought and thought, but he didn't realy have a clue, and finally gave up.
'It's me!' said the barman.
'Er ... oh - ah - yes! Very good! Very good indeed!' cried our hero.

When the colonel got back to England he stayed at his London club for a few days. Chatting to the barman there, the conversation once again turned to riddles.

'I've a good one,' said the colonel, '"It isn't my sister, it isn't my brother, but still it's the child of my father and mother". Well?'
'No idea, sir,' replied the barman, 'who is it?'
'It's the barman at Raffles' Hotel in Singapore!'