Restrict App Access

Sometimes it happens when I'm in a pub, and I'm checking my e-mail, and then a friend asks if he can take a look at my iPhone, it's just such a cool thing!. But I don't want them to access certain apps, like messages, facebook, e-mails, ... Because it's private, and I also don't want to say no, 'cause it's kind of rude, it's like saying: "Sorry mate, I just don't trust you".