Sippy cup.

Sippy Cup chronicles when Beomgyu's mother kills his father and his lover. The bottle itself is metaphorical for the lack of being willing to acknowledge reality, the unwillingness to confront one's own reality.

it was midnight, Beomgyu was 4 years old, he went to the kitchen to get his cup to drink water when he saw that scene. He was supposed to be sleeping under the influence of pills his mother gave him.