Music!
The 'How To Be' actor was asked by the LA Times what makes him cry.
He admitted: "Sometimes music. I very rarely cry about stuff."Or coughing. Coughing makes me cry sometimes" (laughs.)
Rob also revealed that he wanted to do political speech writing when he was younger.
He said: "That's what I wanted to do while I was in school, yeah. I just liked the whole idea of it. I wanted to be involved with politics."That's what my whole plan was. I was going to go to university and then I just thought, 'Ah, I can't be bothered to do anything' (laughs). 'I don't want to do any more homework!'"
source.