What's the Catch?

A catch is a hidden problem in something that looks or sounds perfect. So for example, I saw a sign outside today that says free food all day. Yeah, but what's the catch? Okay, there is a catch. No, except with BBC Learning English, which is actually free. There is no catch. Let's take a look at our next headline. Diet rich in vegetables, whole grains and beans, cuts bowel cancer, risk.