Rule #1: Don't steal the Doctor's sonic screwdriver and hide it 'just for jokes'.

"Rose!" The Doctor pulls open the door to Rose's room and glares down his nose at her.

"Yes, Doctor?" Rose asks, putting the book she is reading aside and looking back at him innocently.

"You haven't seen my sonic screwdriver, have you?" he demands, getting straight to the point.

"No," Rose responds, her expression wide-eyed. "Why, have you lost it?"

"Yes!" the Doctor replies, almost pulling his hair out in frustration. "I need it, really badly. If I don't have it then the TARDIS will explode in exactly five minutes and twenty-two seconds!"

"Oh." Rose blinks, and picks her book back up. "That's a shame."

The Doctor storms from the room, and she can hear his footsteps quickening as he runs around frantically trying to find the lost item. As he wanders, she hears him groan, "Five minutes and thirteen seconds!" Snickering slightly, Rose slips her hand behind her pillow and pulls the sonic screwdriver out from behind her, looking at it with amusement.

"We'll leave it another five minutes ten seconds, shall we?" she asks it, giving it a little stroke.

She swears the item squeaks its approval.