Lighthearted...
You want a pint of milk. There is only quarter pint in the current open bottle.
Do you..
A... pour the the quarter pint into glass, drink it, open new bottle and pour three quarters of fresh milk into glass.
B... pour in the quarter and top up with milk from new bottle, so mixing two bottles.
C... leave the quarter in the bottle, open a new bottle and have pint out of one bottle.