The spell she knew was in it, along with dozens of other created ones. She heard the door open and she closed it quickly, taking the book with her, locked it, and ducked past a shelf and hid from the person coming in. When she looked, hoping it wasn't a teacher, it was a student. Now she was nervous, and she made herself invisible, though she didn't move, hoping they wouldn't bump into her. She read her book silently and began to walk the other direction from the person, hoping they wouldn't notice her walking away.
"What cannot be seen, felt, or carried?" Person who gets this right I will donate some money to them.