空气

空 = 工 + 穴 : Mnemonic symbol: an air conditioner

空 = 工 + 穴 : In front of the engine, Karl Koala found some very special cave paintings. Using steel beams he constructs a protective cover. He also installs air conditioning to make the best environment possible to preserve the cave paintings.

Mnemonic symbol: a balloon. The Queen of Hearts is disappointed by her balloons. She expected them to float upwards, but in the space station's bathroom, They float randomly in all directions.