What makes you think so?

What makes you think so?

What has led you to that conclusion or observation? Can be used sarcastically to indicate that someone's remark was very obvious. A: "I don't believe this is going to work." B: "What makes you think so? I've run the numbers multiple times." A: "I don't think this city is very safe." B: "Huh, what makes you think so? The boarded up windows and vandalized shops?"
See also: make, think, what
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

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?
See also: make, think, what
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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