於是

于 = 亅 + 二 : Mnemonic symbol: prepositions (in, at, on, to, ...) are what glue the sentence together. Honey is as sticky as glue, so I'll represent 于 by a honey jar.

Yu the Great (于) was assigned to repair the electronics of the space station's kitchen (Ø2), but instead he was snacking honey from a honey jar (于). Unfortunately his capacitor (二) got stuck in the honey and now he has to use a crowbar (亅) to lever it out.

是 = 旦 + 龰 : Mnemonic symbol: a skull, from "to be or not to be".

Sherlock Holmes (sh) starts his day in the space station's bathroom (Ø4) with a mug of coffee (旦). Holding a skull (mnemonic symbol for 是) in his hands, he asks himself "to be, or not to be" (是). He wonders if he can make the skull come back to life using lots of coffee, and injects coffee from his mug into the skull using a syringe (龰).

是 = 日 + 下 + 人 : Mnemonic symbol: a skull, from "to be or not to be".

Sherlock Holmes (sh) watches the Neanderthal Man (人) trying to climb to the sun (日) on a ladder (下) in the space station's bathroom (Ø4). Sherlock is afraid that he'll open the window, so he puts on a big skull (是) and scares him off.