"If you can't fit your idea on the back of an envelope, it's rubbish," Richard Branson once told me. The Virgin Group founder is on to something. In fact, most of the famous entrepreneurs and venture capitalists who I've interviewed agree that, to be persuasive, you must be able to communicate your big idea in a short sentence--very short. If it fits in a Twitter post, even better.

Recently, I had a wide-ranging conversation with Yahoo Mail creator and Y Combinator partner, Geoff Ralston. The prestigious Silicon Valley startup accelerator has provided seed funding and training for over 1,460 startups including Airbnb, Dropbox, Reddit, and Stripe. Ralston says he works with entrepreneurs to condense their idea into one sentence.

"If a story is too complicated, it doesn't stick," says Ralston. "You have five seconds to get someone's interest. Every investor has a smartphone and will begin checking email. You will lose them really fast. You need a hook. The best hook for a startup is a very quick way of articulating why what you do matters."

Sequoia Capital chairman, Michael Moritz, is worth an estimated $3.2 billion because of his early investments Google, Yahoo, and LinkedIn. He once told me that successful entrepreneurs communicate their vision clearly and concisely. In 1999, Google founders Sergey Brin and Larry Page walked into Sequoia's offices and said, "We organize the world's information and make it accessible." In one sentence, the investors understood immediately.

According to Moritz, "The best summary I ever heard of a business was from Sandy Lerner, the co-founder of Cisco Systems, who, in 1986, when the company had just eight employees, was asked to communicate her company's purpose. She said, 'We network networks.' It sounded deceptively simple, but it served as the company's north star for the ensuing 25 years."

Moritz and Ralston both acknowledge that simplicity is hard work. Crafting a one-sentence summary of your big idea requires that you understand the benefits of your idea, product or service very, very well.

Steve Jobs was a master at condensing the big idea. Jobs' original vision for the Apple computer was simple: "A computer in the hands of everyday people." It satisfies three requirements--brevity, specificity, and consistency.

It's concise: Eight words and thirty-five characters, short enough to fit in a Twitter post.

It's specific: Jobs put a computer in the hands of 'everyday people'.

It was consistent: Jobs took every opportunity to communicate the vision, and he did so relentlessly. He repeated it in ads, interviews and presentations.

Your customers often require some help to understand where your product or service fits into their lives. Sell the benefit quickly and concisely. Steve Jobs always provided a one-sentence description of a product that sold the benefit without getting into the weeds. What's an iPod? "It's one thousand songs in your pocket." What's a MacBook Air? "It's the world's thinnest notebook."

One short sentence can speak volumes. If your idea can't fit in a Twitter post, keep working on it until it can.