PUZZLEBOXX

Mark poured some water into the jug until it was just over half full.  Then, very carefully, Mark tipped the jug (slowly pouring water out) so that the water just covered the tilted bottom of the jug, like this...

"There you are!" exclaimed Mark, "The volume of the space above the water is exactly the same as the volume of the water .... so the jug is exactly half full."