What makes you think so?
1. Lit. Why do you think that?; What is your evidence for that conclusion? Tom: This bread may be a little old. Alice: What makes you think so? Tom: The green spots on the edges. Bob: Congress is in session again. Tom: What makes you think so? Bob: My wallet's empty.
2. Inf. Is that not totally obvious? (Sarcastic.) John: I think I'm putting on a little weight. Mary: Oh, yeah? What makes you think so? Mary (shivering): Gee, I think it's going to be winter soon. Sally (also shivering): Yeah? What makes you think so?