Terezi honestly expected him to give some sort of demonstration or to mime what she should do. Or simply say nothing at all. So when he begins to apply it himself, Terezi sucks in a quiet breath and holds it.

She almost forgets to breathe during that, and when he finally finished and droops like he's been sucked dry of willpower... That's when she finally exhales in a relieved sort of sigh. He did it. It must have been so difficult, but he did it.

"Thank you," she says, smiling at him and taking the powder from his hands to place the cap back on it. "I'm so proud of you. You did well." She hopes that the encouragement will help to reinforce that this was the right thing to do.