介於

介 = 人 + 丿 + 丨 : Mnemonic symbol: 介 looks like a rocket 🚀. Neanderthal Man (人) and Joan of Arc (ji) are preparing a rocket (介) for its start. Between (介) the layers of the outer shell (介) they are stuffing bananas (丿) and dinosaur bones (丨).

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

Yu the Great was assigned to repair the electrics of the space station's entrance, but instead she was snacking some honey from a honey jar. Now her capacitor got stuck in the honey and she has to use a crowbar to lever it out.