You have a leak-proof drum that can hold 50 litres of water. It has a separable lid, like the one you have on barrels.
On Monday, you fill the drum to the brim, the full 50 litres. Then you leave the room and lock it so that no one can reach to the drum while you're gone.
The next day, you come back and add another four litres of water to the drum without taking any out, and not a drop spills, even though no one removed any water from the drum.
How is this possible?