Teach your grandmother (how) to suck eggs
Give advice to somebody about something they know more about than you.
Water off a duck’s back
Criticisms, insults or warnings that have no effect on the person at whom they are directed.
Alive and kicking
To emphasise that someone or something continues to survive and is very active too.
Let the cat out of the bag
Tell people a secret by mistake or because of carelessness
a glimmer of hope
A very small sign that something we hope for may come true.
change your tune
Change your opinion, above all when you think this will give you an advantage.