tion of a phraseological unit offered by Professor A. V. Koonin, the leading authority on problems of English phraseology in our country:
"A phraseological unit is a stable word-group characterised by a completely or partially transferred meaning." 
The definition clearly suggests that the degree of semantic change in a phraseological unit may vary ("completely or partially transferred meaning"). In actual fact the semantic change may affect either the whole word-group or only one of its components. The following phraseological units represent the first case: to skate on thin ice (~ to put oneself in a dangerous position; to take risks); to wear one's heart on one's sleeve1 (~ to expose, so that everyone knows, one's most intimate feelings); to have one's heart in one's boots (~ to be deeply depressed, anxious about something); to have one's heart in one's mouth (~ to be greatly alarmed by what is expected to happen); to have one's heart in the right place (~ to be a good, honest and generous fellow); a crow in borrowed plumes (£ a person pretentiously and unsuitably dressed; cf. with the R. ворона в павлиньих перьях); a wolf in a sheep's clothing2 (~ a dangerous enemy who plausibly poses as a friend).
The second type is represented by phraseological units in which one of the components preserves its current meaning and the other is used in a transferred meaning: to lose (keep) one's temper, to fly into a temper, to fall ill, to fall in love (out of love), to stick to one's word (promise), to arrive at a conclusion, bosom friends, shop talk (also: to talk shop), small talk.
1The origin of the phrase is in a passage in Othello where Iago says:
... 'tis not long after
But I will wear my heart upon my sleeve For daws to peck at.
(Act I, Sc. 1)
2The allusion is to a fable of Aesop.