sucking the empty bottle

It was strange that both EY and HY started to suck the empty bottles after they finished their bottles. At first, I thought they wanted more milk and I would refit the bottle for them. But, they didn't want the milk, they purposely not to till the bottle so that they were just sucking air. EY did a lot more than HY. I think HY did it because she saw EY doing it. Anyway, EY has been sucking the same empty bottle for the past hour and she was still doing it. Maybe she liked the sound of sucking air out of the bottle?!