a situation in which it is impossible to say which of two things existed first and which caused the other.

It's a chicken and egg situation - I don't know whether I was bad at the sciences because I wasn't interested in them or not interested in them and therefore not good at them.

• a cloud on the horizon
• a C-note
• a cock-and-bull story
• a coffee table book
• a cog in the machine
• a cog in the wheel
• a cold fish
• a cold snap
• a Colonel Blimp
• a contradiction in terms
• a conversation piece
• a cool customer
• a cool head
• a couch potato
• a cradle-robber
• a cradle-snatcher
• a credibility gap
• a crock
• a crock of shit
• a cross has to bear
• a cross someone has to bear
• across the board
• a cross to bear
• a crowd-puller
• a cry-baby