Where there's brass there's muck

Andy had found a corroded and dirty pound coin, and put it in some vinegar to try and clean it (ignoring George's insistent advice that "it's white vinegar you need. And salt. White vinegar and salt, that's the thing.") 

However, while it was soaking someone decided to replace the coin with a two pence piece. 

He was genuinely perplexed at first, but as soon as I took his picture he worked out who the culprit was. 

Well, how could I resist such a dad-gag open goal?