"No, no, Harry, that's not how you slice it!"
Hermione came up behind him and put her hand over his. "You want to slice it like this," she said, guiding his knife and his hands into the correct stroke to cut the valerian root. "It's more potent this way."
She couldn't fail to notice his deeper voice when he spoke. "Thanks, Mione," he said.
She snatched her hand back, suddenly incredibly aware that she had still been touching him. Her eyes widened and her mouth formed a little 'o' for a moment before she bustled off, obviously flustered and confused.
