KanjiJLPT N3
Reason

wherefore, a reason

Mnemonic

A lone stick stands in the center of a field, marking the exact origin point. Why here? The reason is buried beneath — an ancient well where water first sprang up. That stick in that field explains the reason everything around it grew.
Additional thoughts
The kanji 由 looks like a field (田) with a stick poking out the bottom, pointing down to the hidden source. Picture the stick as an arrow aimed at the underground reason — the wellspring — that made this particular field flourish.
Quick recall
A stick in a field points to the buried reason everything grows.

Details

The keyword for 由 is reason. This kanji conveys the idea of a cause, origin, or basis from which something arises. It refers to the underlying grounds or rationale behind events, decisions, or circumstances. It carries the sense of "due to" or "stemming from," pointing to the source or motivation that explains why something is the way it is.
On'yomi
ゆ、ゆう、ゆい
Kun'yomi
よし、よ.る