By Carl Zimmer Last fall, my daughter Veronica got an idea for the seventh grade science fair at her school. She’d compare different ways to clean a toothbrush. First she’d take a new toothbrush out of a package and brush her teeth, covering it with her mouth bacteria. Then, she’d clean it with one of three liquids: water, lemon juice, or vinegar. Finally, she’d wipe the brushes on Petri dishes and see how many bacteria grew on them. It seemed to me like a straightforward enough idea. It might fail, but so what? It would still be worth her time.