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