Magical Dancing Duck

A circus owner walked into a bar to see everyone crowded about a table watching a little show. On the table was an upside down pot and a duck tap dancing on it. The circus owner was so impressed that he offered to buy the duck from its owner. After some wheeling and dealing, they settled for ,000 for the duck and the pot.
Three days later the circus owner runs back to the bar in anger, "Your duck is a ripoff! I put him on the pot before a whole audience, and he didn't dance a single step!"
"So?" asked the ducks former owner, "did you remember to light the candle under the pot?"