The phrase what’s the catch? is a retort when one believes tha | Gravity Falls ENG

The phrase what’s the catch? is a retort when one believes that an offer is too good to be true. One may ask what’s the catch? if he believes that a situation or deal is being presented on its best face and that problems or drawbacks are being hidden. For instance, if a stranger approaches you and hands you a hundred dollar bill, you may wonder what you will have to do to repay that stranger, or if the money is stolen. In this case you may respond, “What’s the catch?” The word catch, in this case, means a hidden cost or a hidden trap.

That's oughta do the trick!