Tern
From Tardis Wiki, the free Doctor Who reference
A tern was a kind of bird.
When a man attempts to capture the bird-like Mr Pakoo, saying "one good turn deserves another", the Tenth Doctor quips "if he's a good tern I'd hate to see a bad one". He then explains the joke to Rose, clarifying that he meant 'tern' as in the bird. (COMIC: Reunion of Fear)
