二十世紀

二 = 一 : Mnemonic symbol: a capacitor, because the two strokes remind me of the symbol in electrical drawings. Albert Einstein (Ø) is experimenting with a capacitor (二) in the space station's bathroom (Ø4). He finds that it's uncomfortable to discharge a capacitor using two flutes (一).

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).

世 = 廿 + L : Mnemonic symbol: the WTO or WHO.

Sherlock Holmes (sh) is cleaning a flower pot (廿) under the shower of the space station's bathroom (Ø4) with a loofah (L) because he didn't quite understand a poster next to the shower from the WHO (世) explaining dangers arising from different germs.

In this case, L is for loofah.

纪 = 己 + 纟 : Joan of Arc finds the space station's bathroom full of silken threads. She even cannot accomplish much using a butcher's hook. She knows that the silk worms are responsible and thinks that soon it will be the era of the silk worms.