One day, a group of 3 criminals escaped from a prison. They sent a police squad to chase after them. Eventually, the 3 criminals went inside a warehouse and they hid in 3 barrels. When the police squad came, they went to the first barrel. The first criminal, thinking fast said ''tweet, tweet''. And the police squad said ''Just a bird''. When they went to the second barrel, the second criminal said ''meow, meow''. And the police squad said ''Just a cat''. When they went to the third barrel, the third criminal said ''bark, bark''. The police squad immediately shot the third barrel. How did they know?