ADJ

broken, empty, hard, outer, protective, thick (figurative)
She had built up a protective shell of indifference around herself
egg (also eggshell)
cockle, conch, cowrie, fossil, mollusc, mussel, oyster, scallop, sea (also seashell), snail
The garden was littered with empty snail shells
coconut, walnut

VERB + SHELL

have
creatures that have shells
come out of, emerge from (often figurative)
He's really come out of his shell since he met Marie
go (back) into, retreat into, withdraw into
The snail went back into its shell
remove sth from
Remove the mussels from their shells

ADJ

heavy
unexploded
anti-aircraft, artillery, cannon, howitzer, mortar

VERB + SHELL

load
fire

SHELL + VERB

fall, land
blow up, burst, crash, explode
hit sth, strike sth
Two shells hit the roof
blow sth apart/off, blow sb/sth up

SHELL + NOUN

fire
They braved heavy shell fire to rescue the wounded
case/casing, fragments, splinter
crater, hole

ADJ

concrete
burnt-out, empty, hollow
The fire reduced the school to a hollow shell