If some person asks you about your exams you should not reply with

„They have been a piece of cake“

Why that? Well, you could get another questions like

„How many pieces of cake?“ or

„Was it a big piece of cake or a small one?“.

If the exams went well, you want to impress and answer confidently you have to say:

„I killed the paper!“

This is also an universal statement everyone understands and not a British Saying … Thanks to Mohammed for this great lesson late at night in our living room ;)