Red Devil and something to think about

Attempted Devil by Jun Maekawa... its an indication of an advanced folder... tough model...but handled it relatively well... super detailed model, it even has fingers and tongue...
















Anyway here is one problem for those with nothing to do to think...
A tank is 30cm x 30cm x 30cm, completely filled with water. There is another perfect cube, 10cm x 10cm x 10cm. How can one fill exactly half the cube with no other equipments whatsoever... Think out of the box... haha