Which iphone is the thinnest?

My brother has just come home from visiting my grandparents and he has a phone. He says it is an iphone, but it looks like an ipod. Since my brother won't let me take it out of its case I can't tell, but I do know his supposed phone is really thin, reminds me of an ipod 4gen. In order to figure out what iphone it is I need to know which iphone out of the 1st iphone, 2nd iphone, and 3rd is the thinnest. Please help me… I'm really curious and want to know.

There's a few ways to tell. If it has iOS 7 it must be an iPhone 4, iPhone 5, or iPhone 5s then lastly an iPod Touch 5th generation. Also to tell if it's an iPod 4 or 5 is if it's only black or white and still had a curved back that is silver and shiny, it's an iPod 4. Out of the first 3 iPhones which are iPhone 2G, iPhone 3G and iPhone 3GS then the max iOS will be iOS 4,5, or 6. Then lastly the iPhone 3GS is the thinnest of those but the thinnest overall iPhone is the iPhone 5 and 5S. Overall thinnest device though is the iPod touch 5G!

Well if it's an iPhone then it should have something called cellular data in the settings and the phone at the home screen to make calls iPods don't have that so check and you'll find out