Lirael looked at the small silver object. It was a mouse with little mechanical legs, two bright chips of ruby for eyes, and a small key in its back. She could feel the warmth of a Charter-spell lying dormant inside it. She supposed that this would activate the clockwork mechanism at the right time and send it wherever it was supposed to go.
"What's it do?" Lirael asked [...].
"It gets help," replied Imshi. "If you're in the Old Levels or somewhere you don't think anyone will hear the whistle, put the mouse on the ground and speak or draw the activating mark, which I'll show you in a moment. Once it's activated, it'll run to the Reading Room and sound the alarm."

Lirael, Ch. 6, pg. 53-54