He adds his hair.

Harry only hesitated for the briefest of seconds before adding a single hair of his into the potion, the potion brightening momentarily with the addition. He thought maybe Hermione was a bit attracted to him, but he did not want to entirely depend on his ability to seduce her once the potion started working. Making sure Hermione was fully engrossed in her book, he dumped the potion into her goblet and left the hall hidden under the cloak.

He joined her a minute later and sat across from her, she smiled at him as she raised the goblet to her lips. Harry mentally patted himself on the back watching her take a good swallow of the pumpkin juice.

As she lowered the goblet, Hermione looked at it in puzzlement and Harry froze at the gesture.

"Everything alright there Hermione?" Harry asked hoping he sounded calm.

Hermione shook herself out of her musing and looked at him, "I dunno, the juice just tastes different for some reason."

Harry fought to keep the panic off his face "Really, Different how?"

In response she took another sip before answering, "Sweeter, more refreshing maybe. Honestly it is like the best juice I have ever had."

"Maybe it is a fresh cask, used different pumpkins, could be as simple as you being in a good mood and it just tastes better."

Hermione thought it over for a second before nodding acceptance, and taking another drink. "You're probably right, I have been happier since we started our last year. Just a normal year with normal worries." Hermione's eyes softened and he thought her voice took on a huskier tone as she reached across the table and took a hold of his hand, "Maybe we'll even get a chance to get up to whatever normal students do?"

Harry only nodded at her as she winked at him and continued to drink the potion laced juice. He wondered if Hermione's playful behavior was her or if the potion was already taking effect.

Harry discretely glanced at Hermione's breasts as she tipped the goblet back, trying to see if there was already any growth.