:up: The other thing in favour of 'can' is that it emphasizes the idea of doing as much as you can..
Another idiom - similar in some ways (emphasizing the limits of what can be done) but not meaning the same - is 'You can take the <noun> out of <place> but you can't take <place> out of the <noun>' - which refers to the influence of the place you're born in on who you are: a non-PC example is 'You can take the girl out of Essex but you can't take Essex out of the girl'.
"This idiom appeared in literature over the centuries in a variety of forms;for example, in the play Narcissus, which was published in 1602, of unknown authorship, subtitled as A Twelfe Night merriment, played by youths of the parish at the College of Saint John the Baptist in Oxford:
Your parents have done what they coode,
They can but bringe horse to the water brinke,
But horse may choose whether that horse will drinke. "
As a EFL teacher,I can interpret this idiom by saying that we cannot force a child or put pressure on him to learn if he is not willing and ready to do it.
"People, like horses, will only do what they have a mind to do"
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