My favourite guinea-pig feature...

is the shape a guinea pig's mouth forms when it looks up - hanging open slightly, with just a hint of teeth. A guinea pig always looks as though it's smiling when it does this. I'm not sure if the mouth is always in this form because I see it only when I chuck a chin or hold a piece of tasty vegetable above a snout.

This favourite feature is modelled by my oldest guinea pig, Trick, who is over 6 years old. She has runny eyes and a runny nose, and is extremely skinny for no other reason except that she's elderly. Trick's a great character, a real bruiser of a boss pig - she sleeps most of the time these days, but still eats her weight in cucumber when she can.