In the book Hoot, how did Roy's bike get a flat tire?

In the book Hoot, Roy's bike tire gets a flat because Beatrice bites a hole in it.  

At the end of chapter six, Roy rides his bike to the construction yard. He is looking for Mullet Fingers, because Roy wants to show him that he can be trusted.  Roy doesn't find anything, and when he comes back to retrieve his bike, it is gone.  Roy starts walking home. 

Chapter seven begins, Beatrice shows up, and she is riding Roy's bike. She orders him to get on the handle bars, and she continues to pedal. Beatrice is really strong. The two of them stop and talk about Mullet Fingers.  Beatrice eventually reveals that Mullet Fingers is her brother. 

Roy has now been out longer than he should have.  In order to explain his tardiness, Beatrice comes up with idea that Roy had a flat tire.  To make it flat, she bites it.