[danieltillett]

No business idea that can be pitched in one sentence can be brilliant. Any idea that can be shortened down that much is not brilliant, which is why so many pitches are of the kind "We are an ex-Google/Facebook/Amazon team building the Uber/Airbnb/Facebook for niche x". Yawn.

Truely brilliant ideas are very, very rare and take quite a long time to explain.

Edit. My approach is to say that I have solved problem x. If the person is interested in this then I then ask if they want to learn more and tell them approximately how long it will take to explain. Most people aren't interested, but for those that are it is always a productive and enjoyable experiences to talk to them about the idea in depth.