The mind boggles. - something that you say which means that a situation or subject is very difficult to understand or imagine.

His latest book is a mixture of physics, astronomy and philosophy - all mind-boggling stuff.

• You can't keep a good woman down.
• You can't make an omelette without breaking eggs.
• You can't make a silk purse out of a sow's ear.
• You can't make bricks without straw.
• you can't miss it
• You can't say fairer than that.
• You can't teach an old dog new tricks.
• You can't win 'em all.
• you could cut the atmosphere with a knife
• You could have heard a pin drop.
• You could have knocked me down with a feather!
• You could have knocked me over with a feather!
• you could hear a pin drop
• you couldn't have asked for anything more
• you couldn't have asked for more
• You could've fooled me.
• You'd better believe it!
• you don't need this
• you have got to be kidding
• you haven't lived
• you haven't lived until
• you haven't lived until something
• You have to be cruel to be kind.
• You have to hand it to
• You have to hand it to someone