Idioms are fixed phrases that are normally not subject to alteration, proverbs being the longest of such constructions. Any of the components of idioms may have a set of (near-) synonyms, but these semantic alternatives are not available for substitution in idiomatic expressions. Thus, one says “break a leg,” but not “break a foot,” when one intends the to wish someone good luck.
Apropos, words that name the parts of the body are particularly frequent in idioms in all languages. A generalized reference to the head in American English, for example, can be made by using head, mind, brain, cranium, skull, noggin, noodle/noddle, pate, etc. But the contemporary idiom “get one’s head around” cannot be altered, though one occasionally hears even native speakers mistakenly tampering with it in utterances recorded by the broadcast media.
Parenthetically, the professional linguist’s injunction to “Leave your language alone” not only encourages users to turn a deaf ear to prescriptivism but may also license a linguistic freedom which turns a blind eye to error.