take the bull by the horns - to take action without worrying of the results
ex. He took the bull by the horns joining the contest even if he does not sing well.
tail between one's legs - to feel ashamed especially after a scolding
ex. He left the house with his tail between his legs, after his mom scolded him in front of his peers.
smell a rat - to be suspicious or feel that something is wrong
ex. She is too nice to them, considering this is the first time she is supposed to have met them. I smell a rat.