Heidi could see through Tracey's face that she was thinking. Even more so that she was now more determined than before. And seeing also that her apologies were getting her nowhere, Heidi stopped them, saving her breath.
Heidi moved her eyes away from the blonde Slytherin, to the rest of the room. She saw their condescending stares and their giggles and chuckles at her expense. She was, of course, more used to this that Tracey, having messed up many more times, but they still got to her. She was now determined too, to make a better potion, even if she as only helping.
Heidi chastised herself for being angry with Tracey. It really was her fault, and she supposed that Tracey had a complete right to be angry. Now, her anger switched to herself. How could she have done such a stupid, careless thing? Of course the bloody powder needed to be measured! She dug her fingernails into her palm as her face remained calm.
Nodding silently, Heidi picked up the knife, cautiously enough to make sure they were near perfect. Silently she worked, mentally and almost physically shrinking back in embarrassment and self-indignation. She wanted to say something, anything, but feared it might only serve to make Tracey madder.