Phineas and Flies

Ferb has gone to visit the dentist so Phineas is now alone at home. But as usual he was thinking what he can do for the day. He was just looking at the sky through the window but unfortunately some flies have settled on the window and are obstructing his view. Phineas immediately hurried to get his square racket for killing flies. At the same time, he wondered how many flies it is possible to kill