Disclaimer: I do not claim to own Naruto. If I did, it wouldn't be nearly as popular as it is now.
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Sakura stopped by the interrogation office on her way back to the shared apartment. She greeted the guards and was allowed access inside. When she asked to see Deidara's records, she answered their questions by telling them that it was for interrogation purposes. She wanted to get closer to him in order to get more information out of him. What little knowledge they had on his background would help.
They seemed a little skeptical but let Sakura have access to his records anyway. She was in. Now she just had to distract them as she took the clay.
"Whoops!" Sakura let one of the folders slide out of her hands and into the container with his confiscated items and weapons. "Sorry about that. I'll get it." She managed to scoop up some clay into the folder. Now if anyone asked, she could always pretend it was an accident.
Sakura messed up some of the other items in her ploy so that it would be harder to tell that some of the clay was missing. She straightened up, folder in hand, and asked "Can I take this with me?"
When asked "Why?" she answered, "I think I might be able to dig up some new information to go in it. Besides, it's not like it contains any information Deidara doesn't already know."
She received permission. Everything was working out fine.
That evening Sakura asked Deidara if he could teach her. "What can I teach you without my clay, un?" Sakura bent over to take something out of a folder. "My clay, un!"
"Perhaps we should hold off on any explosions." When Deidara gave her a questioning look, she answered with the fact that they didn't want to draw any unnecessary attention from the current lookouts.
"First, you're going to have to take those gloves of yours off, un." She complied and he winced at seeing her stitches. A chakra scalpel, a few tugs and a little healing later and the stitches were gone. Her mouths were there for all the world to see.
"Now we can start your training, un."