"I like how tight they feel," you answer, blushing. "And I want to see what happens if..."
"If what?" the voice prompts.
"If I keep them on."
"Well, between you and me, they're going to get a lot, lot tighter, dude. Your shorts especially." The speaker pauses. "But you know that, don't you?"
"Wha- What do you mean?" Your heart is beginning to pound; your face feels hot.
"I know what you're thinking about." The man sounds amused. "Boys bending over... changing... Sprouting tails through the seats of their pants."
"Awwww..." The way he said 'tails' makes you moan.
"Donkey tails... that whip around behind them." The speaker is clearly enjoying himself. "You want that too - don't you, dude?"
"Yes!" you gasp, rubbing your backside in the region of your tailbone. "I want to sprout a long donkey tail right through my shorts!"
"Then bray, dude," the voice encourages. "Bray like a jackass and bend over."
"Awwww," you moan again, excited beyond all measure. Your whole body is tingling, your heart is thumping, and at the top of your backside a pulse has started up. "It's going to happen - I'm going to do it!" you exclaim, gripping your buttocks with both hands. "I'm going to sprout a tail! A long, donkeee Hawwwwwww!!!"
The bray bursts from your lungs, rich and realistic, and at the same time you bend over, your shorts pulling tight around your ass.
"That's it," the voice says. "Now grip your knees and push!"
"Heee Hawwww!" you bray again, doing as he instructs. The pulse in your butt builds in strength - a bump is beginning to form. "Oh yes!" you cry as you reach back to touch it. "I've got a tail growing in my shorts!!!"