Du ställer frågor till de personer du samspelar med det bara de skulle veta. Detta händer några gånger i de senare böckerna. Här är ett citat efter slaget av de sju Potters.
No sooner had Mrs. Weasley bent over her son that Lupin grabbed Harry by the upper arm and dragged him, none too gently, back into the kitchen, where Hagrid was still attempting to ease his bulk through the back door.
"Oi!" said Hagrid indignantly, "Le' go of him! Le' go of Harry!" Lupin ignored him.
"What creature sat in the corner the first time that Harry Potter visited my office at Hogwarts?" he said, giving Harry a small shake. "Answer me!"
"A – a grindylow in a tank, wasn't it?"
Lupin released Harry and fell back against a kitchen cupboard. "Wha' was tha' about?" roared Hagrid.
"I'm sorry, Harry, but I had to check," said Lupin tersely. "We've been betrayed. Voldemort knew that you were being moved tonight and the only people who could have told him were directly involved in the plan. You might have been an impostor."
"So why aren' you checkin' me?" panted Hagrid, still struggling with the door.
"You're half-giant," said Lupin, looking up at Hagrid. "The Polyjuice Potion is designed for human use only."