In the minisode Time Crash (available in full on Youtube), the Tenth Doctor says to the Fifth Doctor:
Oh no, of course! You mostly went hands free, didn't you? Like 'eh, I'm the Doctor, I can save the universe using a kettle and some string, and look at me, I'm wearing a vegetable'.
This refers to the fact that the Fifth Doctor didn't use a sonic screwdriver for most of his tenure on the show. But is it a reference to anything more specific? Is there actually a Fifth Doctor episode where he saves the universe using "a kettle and some string", or is Ten just inventing a random example of everyday household objects?