Harry lies about why he made the potion.

While Harry did trust the professor with her life, she didn’t exactly want to admit what she’d been doing with her Polyjuice Potion, especially why. After all, Ron was in the room, and still had no clue that Harry had made the potion in her lust for Ron’s younger sister, Ginny. So, Harry lied as best she could. “I was making some Polyjuice Potion, sir, to practice for the next term. It was going well, but I ended up adding some extra ingredients: petroleum jelly and Hungarian Horntail scales. When I drank it, it didn’t turn me into the girl whose hair I put into the potion, but did...this to me,” Harry said, gesturing to her changed body.

For a few tense seconds, the Hogwarts Headmaster said nothing in response, only looked at Harry seriously. Then, he finally replied, with a knowing twinkle in his eye, “Ah, I see. Most likely some pretty Muggle girl in your neighborhood, Harry?” Jumping on the excuse, Harry nodded fervently, and Ron didn’t seem to realize what they’d just covered up. “That was quite dangerous, in two different regards, Harry.

“First, in that making that potion came quite close to breaking the rule against Underage Magic, in a way that I never could have protected you from.” Harry froze, never having thought about that possible consequence before. Still, Dumbledore seemed to be in good humor. “That said, you didn’t case any actual spells, and the making of a potion isn’t something the Ministry for Magic can check as they can normal spellwork.”

Now, Harry felt like a proper fool. If she had made a single misstep in her potion, she would have likely been expelled from Hogwarts, all over her teenage hormones. Hesitantly, Harry asked, “And, what was the second dangerous thing, Headmaster?” Harry’s heart was beating loudly underneath his feminine chest.

The smile retreated from Dumbledore’s face, leaving his countenance grim once again. “Adding those extra ingredients to the recipe. Potion work is an incredibly precise practice, Harry. The reason the recipes are set in stone is because altering them in the wrong way can lead to radically different effects. For example, how long has it been since you drank the potion?” Harry counted it out on her thin fingers, and quickly saw the problem. “Indeed. Your change in ingredients has drastically changed the result.”