Two iPhones How do you tell the difference between them?

Here’s the thing. The Wife and I are getting an iPhone each. Apart from turning them on and seeing which background image is on there, how can you instantly tell them apart. I thought that engraving might be the best way to go for this. So, I’m planning on getting mine engraved with an odd saying or something (rather than my name or something like that). I recently heard an interesting statement: Goldfish don’t bounce. That’s the best I can come up with.

Any other amusing ideas?

UPDATE: It seems Apple don’t offer engraving options for the iPhone. At least not at the moment in the UK.