I like Speedwell's explanation. In that case an exam is like a battle, if we win (pass) it we have our colours (flags) flying, if we don't we end up as losers.

Oops, I've made a mistake. A small correction will be needed here: 'mouse potato' isn't glued to the TV screen but only to computer one. Isn't that phrase for a computer freak nice? what do you think about it? These 'potato' expressions have been making amazing career in Polish language. Couch potato has already been calqued into Polish as 'sitting potato'. I've come across it in one of the student magazines and it appeared in the phrase 'the epoch of sitting potatoes'.

't' and 'j' are pronounced as 'ch' when they are on the margins of the words e.g. in 'don't you'. Linguists call it 'palatalisation' and it's a kind of phonetic 'trick'. Similarly, it happens between 'd' and 'j' in 'd'you like it?'. Then it gives the sound of 'g'