Upper trigram
☲
Fire · Li
Clarity and attachment.
FamilyMiddle daughter
ElementFire
Hexagram 21 of 64
☲☳ Biting Through
Decisive action against an obstruction.
Lower trigram
☳
Thunder · Zhen
Arousal and shock.
FamilyEldest son
ElementWood
The image
Fire above, Thunder below.
Fire over Thunder: clarity and attachment meeting arousal and shock.
That combination is the whole meaning of hexagram 21. Nothing about it is arbitrary — change either trigram and you get a different hexagram with a different reading, which is why the 64 form a system rather than a list.