A guy walks into a bar, and he's got a duck on a biscuit tin. Barman says, "What's that?" Guy says, "It's a dancing duck, watch." He puts the duck on the biscuit tin on the bar and after a few minutes the duck stands up and begins to bob from one foot to the other, just like it was doing a little dance. "That's fantastic!" says the barman. "That'd make a great talking point for my pub! I'd like to buy it!" They agree a price and the guy goes off.
The next morning, the barman opens up, and he proudly puts the duck on the biscuit tin on the bar and waits for it to start dancing. Nothing, it just sits on the biscuit tin on the bar. Barman tries everything he can think of to get it so dance: feeds it, gives it some water, plays it some music. All to no avail, it just sits there on the biscuit tin on the bar.
Barman's right gettin' the hump now, just when the guy walks in again. "Oi!" says the barman, "This duck you sold me's a dud, it doesn't dance!" The guy looks surprised and says, "Hang on, let me have a look." So he takes the duck off the biscuit tin and puts it in the bar, then he takes the lid off the biscuit tin and has a look inside.
"Oh, no wonder!" he says, "You forgot to light the candle."