Left: armor, shell 甲 (The seedling丨 cracks its shell/husk 田), right: bird 鸟/鳥 (= with head 日, tail フ, and claws 灬))
Hard as an armor is (the beak) of this bird: The duck.

Left: hand 扌, right: shell/armor 甲 (The seedling 丨 cracks its shell/husk 田)
The hand pushes into a "shell" when: detaining.

It is shown a root-seedling 丨 with its shell/husk 田
The seedling cracks the shell. (but with a reason 由 which grows 丨 upwards.)

Left: framing 匚, right: armor, shell 甲 (The seedling丨 cracks its shell/husk 田)
A framing needs a certain strength (=armor) to serve as a box.

Outside: gate 门/門, inside: armor, shield 甲 (The seedling丨 cracks its shell/husk 田)
(It can be open or closed) The gate is like a switchable shielding, switchable just like: the gear of a gearbox.

Left: metal 钅/金, right: armor, shell 甲 (The seedling丨 cracks its shell/husk 田)
(Use as fertilizer) The metal that cracks the shell of the seedling: is potassium.