[He] ain’t got the brains God gave a squirrel or ain’t got the sense
God gave geese.
It seems your friends may be struggling with idiom overload:
- "Ain't" is a very informal replacement for "doesn't".
- "Got" is an informal expression for "have".
- "Brains" is idiomatic for intelligence.
Here is a simpler version to start with:
His brain is smaller than a squirrel's, so he doesn't even have the
sense of a goose.
Then you can explain one idiomatic expression at a time, comparing the meaning to the simpler version, until they capture the original phrase.
A way to help them understand the metaphorical use of "take a shine to":
- When someone you like walks into the room, you smile brightly, and we say your face "shines" (like the sun shines).
- When you like someone you are just getting to know, you smile with that same shining face, you greet them cheerfully, you are very kind to them, and you do your best to impress them with your best qualities. We say you are "shining up to" them" (like you polish your silverware to welcome an important guest).
- When you like someone, and we can all see you are "shining up to them", we say "you have taken a shine to them" (like you are the silverware that has been polished).
- In all three cases, we are talking about how you behave and feel in terms of a pleasant shining light:
1520s, "brightness," from shine (v.). Meaning "polish given to a pair
of boots" is from 1871. Derogatory meaning "black person" is from 1908
(perhaps from glossiness of skin or, on another guess, from frequent
employment as shoeshines). Phrase to take a shine to "fancy" is
American English slang from 1839, perhaps from shine up to "attempt to
please as a suitor." Shiner is from late 14c. as "something that
shines;" sense of "black eye" first recorded 1904.
Old English scinan "shed light, be radiant, be resplendent,
iluminate," of persons, "be conspicuous" (class I strong verb; past
tense scan, past participle scinen), from Proto-Germanic *skinan
(cognates: Old Saxon and Old High German skinan, Old Norse and Old
Frisian skina, Dutch schijnen, German scheinen, Gothic skeinan "to
shine, appear"), from PIE root *skai- (2) "to gleam, shine, flicker"
(cognates: Sanskrit chaya "brilliance, luster; shadow," Greek skia
"shade," Old Church Slavonic sinati "to flash up, shine," Albanian he
"shadow"). Transitive meaning "to black (boots)" is from 1610s.
Related: Shined (in the shoe polish sense), otherwise shone; shining.