to introduce / to lie between / between / shell / armor
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.
(of time or place) in; at; on / (indicating any indirect relation) to; toward; vis-à-vis; with regard to; for / (indicating a source) from; out of / (used in comparison) than / (used in the passive voice) by