"In bocca al lupo" is an idiomatic way of saying 'good luck to you', and it
is used when somebody is undertaking something difficult, like
going alone on mountain climbing, or taking an examination;
it has an idiomatic answer: 'crepi il lupo', that is: let the wolf die;
'crepi il lupo, grazie' is also ok, but 'grazie' alone is not ok.