fit/write (something) on the back of a postage stamp

if you say you could write what you know about a subject on the back of a postage stamp, you mean you know very little about that subject.

What I know about car maintenance could be written on the back of a postage stamp.

• corner the market
• corridors of power
• cost an arm and a leg
• cost a pretty penny
• cost dear
• cost someone an arm and a leg
• cost someone a pretty penny
• cost someone dear
• cotton-picking
• cotton to
• cotton to someone
• cotton to something
• couch potato
• cough up
• cough up something
• could do something in sleep
• could do something in their sleep
• could do something standing on head
• could do something standing on your head
• could do something with eyes closed
• could do something with eyes shut
• could do something with one arm tied behind back
• could do something with one arm tied behind their back
• could do something with one hand tied behind back
• could do something with one hand tied behind their back