How 3 guys turned renting an air mattress in their apartment into a $25 billion company

Long before Airbnb persuaded strangers to sleep in one another's homes and became a $25 billion company, it was just an idea to make extra bucks and make rent.

After their first guests, Airbnb's founders realized they were on to something bigger than a stopgap for rent. They faced rejection plenty of times - and created their own version of Obama O's cereal - but the three founders of Airbnb have built a big business in the past nine years.

Here's how they turned their idea to rent out an air mattress into a business that has the hotel industry running scared.