土

土 = 十 + 一 : Tommy Turtle (tu) wants to fix a cross (十) on a flute (一) in the space station's living room (Ø3). He uses dust and clay (土) to hold them together.

Maud Younger (y) forgot her keys to the space station (Ø1), all she brought outside was her flute (一). She has no choice but to crash the door with the giant flute.

十 = 一 + 丨 : Mnemonic symbol: a holy cross. Sherlock Holmes (sh-) is investigating a very strange thing he found in the kitchen of the space station (-2): A big pyramid made of ten (十) crosses (十), which are in turn made out of flutes (一) and dinosaur bones (丨).