"Don't say 'it can't be done', say 'I don't know how to'".
It was probably 10th grade, a science class, and it referred to a particular chemical reaction, but it's usable in a surprisingly wide range of contexts!
Actually, it was in Spanish and to me it was subtly different, since the "yo" is optional so including it is emphasizing that it is oneself in particular that does not know:
"No digas 'no se puede', di 'yo no sé'".
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