"You will keep looking [through books for Mr. Flamel] while I'm away [on holidays], won't you?" said Hermione. "And send me an owl if you find anything."
And you could ask your parents if they know who Flamel is," said Ron. "It'd be safe to ask them." (Harry Potter and the Sorcerer's Stone)
Hermione replies to Ron, "Very safe, as they're both dentists": I guess Hermione’s parents can keep secrets about Magic world for they wear masks and their clients have to open wide during services. But is there any British tale or idiom about the keeping secret of dentists?