to exist; to be alive / (of sb or sth) to be (located) at / (used before a verb to indicate an action in progress)
在
=
一
+
亻
+
土
:
Zorro (z) and Rosa Luxemburg (亻) exist (在) in the airplane's bathroom (ai4). Rosa is in the middle of balancing a flute (一) on her head and on a pile of clay (土). Zorro uses his rapier to inscribe "Z" into the flute, so as to mark that the pronunciation of "在" begins with "z".
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
life is motion (popular saying with many possible interpretations) / Physical effort is vital for our bodies to function (Aristotle). / Life derives from physical exercise.