... the hatchet.
Bury the hatchet.
... your tongue.
Bite your tongue.
At the ... of a hat.
At the drop of a hat.
Sit on the ....
Sit on the fence.
Every cloud has a ... lining.
Every cloud has a silver lining.
It's a ... of cake.
It's a piece of cake.
Burn the ... oil.
Burn the midnight oil.
Bite off more than you can ....
Bite off more than you can chew.
Don't put all your eggs in one ....
Don't put all your eggs in one basket.
Curiosity ... the cat.
Curiosity killed the cat.
