Verb
get blood out of a stone (third-person singular simple present gets blood out of a stone, present participle getting blood out of a stone, simple past got blood out of a stone, past participle (UK) got blood out of a stone or (US) gotten blood out of a stone)
(idiomatic) To do something difficult, frustrating, or pointless.
Convincing fussy eaters to try new foods is like trying to get blood out of a stone.