1) Open A Can Of Worms-to create a situation that will cause trouble or be unpleasant-
The investigation into how those expensive trips were paid for certainly opened a can of worms.
2) Strike While The Iron Is Hot-to act on an opportunity promptly while favorable conditions exist-
He doesn't often make such offers - I'd strike while the iron is hot if I were you.
3) Put Your Foot Down-to use your authority to stop something happening-
When Monica wanted to spend all the savings on their wedding, Chandler had to put his foot down.
4) Wild Goose Chase-a worthless hunt or chase, a futile pursuit-
Physicists searching for the hypothetical particle may be on a wild goose chase.
5) Throw Caution To The Wind-to do something without worrying about the risk or negative results-
You could always throw caution to the wind and have another glass of wine.
6) See Eye To Eye-be in full agreement-
The boss and I do not always see eye to eye, which often creates problems.
7) Scratch The Surface-to deal with only a small part of a subject or a problem-
The amount of aid that has been offered is hardly going to scratch the surface of the problem.
8) Once In A Blue Moon-not very often-
My sister lives in Alaska, so I only see her once in a blue moon.
9) Pour Your Heart Out-to tell someone your secret feelings and worries-
She finally broke down and poured her heart out to him.
10) Run Of The Mill-lacking unusual or special aspects, ordinary-
He wasn't too happy with his run of the mill job.
11) Leave No Stone Unturned-to do everything possible in order to achieve or find something-
He left no stone unturned in his search for his natural mother.
12) Learn The Hard Way-to learn something by an unpleasant experience-
I found out the hard way that it's difficult to work and go to school at the same time.
13) A Blast From The Past-something that suddenly makes you remember an earlier time in your life-
Hearing that record again was a real blast from the past.
14) A Blessing In Disguise-an apparent misfortune that eventually has good results-
Losing that job was a blessing in disguise for him, for he found a better one.
15) With Flying Colors-with great success-
She took a driving test and passed with flying colors.
16) Out Of The Blue-something that is completely unexpected-
One day, out of the blue, she announced that she was leaving.
17) Count Your Blessings-to realize your good luck-
Seven climbers are counting their blessings after surviving a sudden snowstorm yesterday on Mt. Rainier.
18) Add Fuel To The Fire-to make a problem worse-
Shouting at a crying child just adds fuel to the fire.
19) See The Light Of Day-to be made available or be known about, appear for the first time-
The company agreed the ad was an embarrassment and promised it would never again see the light of day.
20) Pull A Fast One-to deceive or trick someone-
You paid too much, I think he pulled a fast one on you.