The water bottle and the milk carton are to keep time, everytime he goes out to the store to buy food, the water from the milk carton drips into the empty water bottle to keep time and when 2 cells are filled it indicates time is up and sunghos back.
What happened in the past is that they where secretly fucking and then shit happens can't say more cause spoiler.
Wow, thanks! The water thing was driving me crazy
"The water bottle drips serve as a timer for counting down as to when Sungho will be coming back"
An Anonymous said this twice in the other same questions below, and other people replied that too.
Lol yea, it's a good way to keep time, but becomes unreliable as time goes by, cause if he doesn't switch the milk carton with hole, the water will slowly deteriorate the open space of the milk carton and water will drip in faster, I love this stuff xD
Damn sungho just get the poor guy a clock! Or an hourglass would do
Lmfaooo
first off, whats the thing with the bottle of water? why does it keep dripping? and also, i didnt get what dohyun did to sungho in the past and evidently nobody does.