You fold your arms in disgust.  "No, I don't happen to have a pony in my backpack, or anywhere else for that matter.  Now, enough of this nonsense!  I demand that you let me cross this bridge!" 

The troll draws up to his full nine-feet-one-inch height (he's a young troll, and still growing -- and trolls grow fast!). 

"You'll have to fight me to get past me!" he bellows. 

"Easy enough!"  You draw your sword and charge. 

"I wouldn't give you the pleasure!" You storm away, hoping to find another crossing downriver.