The weekly free paper R25 published an interesting trivia factoid recently (Issue 247, but the article doesn't appear on their website), about why we say "say cheese!" when taking photos.
According to them, the original expression in America was "say cheers!" When US servicemen stationed in Japan took photos, however, their Japanese subjects' attempts at saying 'cheers' came out sounding like 'cheese'. The soldiers began using 'cheese' deliberately, the expression caught on back home, and is now the standard everyone uses today.
Truth or Bullshit?