Light-bulb moments don’t happen on command, and brainstorming sessions rarely produce extraordinary results. More often it’s a random remark, event, or memory that sends an entrepreneur down the rabbit hole of innovation. From Airbnb to Yelp, here are the surprising origin stories to eight of today’s hottest companies:

1. A Study Abroad Photography Class Sparked The Idea For Instagram

A study abroad opportunity during his junior year at Stanford University provided Kevin Systrom with the inspiration for Instagram. Systrom brought his advanced SLR camera to the photography class he was taking in Florence Italy, but his teacher replaced it with an inexpensive Holga that used random light leaks and vignettes borders to produce interesting photos. Systrom loved the aesthetic: “It taught me the beauty of vintage photography and also the beauty of imperfection,” he told Forbes magazine in 2012.

A few years after graduating in 2006, Systrom and partner Mike Krieger were working on developing a photo-sharing app. While on a vacation, Systrom’s girlfriend, Nicole Schuetz, remarked that she would be reluctant to use the app because her pictures weren’t as good as a mutual friend’s. Schuetz thought it was due to the friend’s eye for photography, but Systrom knew it was his use of filters, prompting him to remember his experience in Florence. That day, he designed the first Instagram filter—called X-Pro II—that let users turn ordinary photos into hip, artistic images. Instagram was launched in October 2010; a month later it had one million users.

2. A Lonely Childhood With A Police Scanner Started The Idea For Twitter

The inspiration for Twitter came from cofounder Jack Dorsey’s childhood fascination with a police scanner. A shy child growing up in St. Louis, Dorsey told 60 Minutes he preferred to stay inside and listen to the police scanner instead of play with friends. He was struck by the short bursts of information shared by law enforcement and emergency personnel.

“They’re always talking about where they’re going, what they’re doing, and where they currently are,” Dorsey said on the CBS news program. When cell phones and text messages became popular ways for friends to update each other on their whereabouts, Dorsey remembered his days listening to the scanner. “And that is where the idea for Twitter came,” he said. The company was created in 2006.

3. A Failed Stock Purchase Started eBay

In 1995, Pierre Omidyar, founder of eBay, had some extra cash and wanted to invest in the IPO of a gaming company. The opening price was set at $15 per share, and Omidyar called his broker to place the order. Later, his broker informed him that he purchased the stock for $24 a share, explaining that $15 was the “ideal” price, not the price the “regular people” could get.

“The takeaway was that the theory of efficient markets is really great—in theory. In practice, regular people are locked out,” Omidyar told Inc. magazine in 2013.

Omidyar decided that the Internet could be the equalizer, bringing the power of financial markets to the public: “Of course, regular people aren’t selling stocks in their households; they’re selling stuff,” he said in the interview. “I thought, ‘There’s a real opportunity to create a marketplace that could bring the power of efficient markets to regular people.’” He launched eBay that Labor Day.

4. A Late Fee For Apollo 13 Led To The Idea For Netflix

“Houston, we have a problem.” This sentence not only rescued a group of astronauts in 1970; it prompted the idea for Netflix.

Founder Reed Hastings had rented the movie Apollo 13 and returned it well past its due date: “I had misplaced it and it was six weeks late, so it was a $40 late fee,” he told 60 Minutes. “I remember ’cause I didn’t want to tell my wife. … I knew what she would say.”

On the way to the gym, he wondered why video stores didn’t operate like health clubs, charging a flat membership fee. The light-bulb moment prompted Hastings to buy a bunch of DVDs and mail them to himself to see if they would travel safely through the mail.

“I opened them up, and they were fine,” he said in the interview. “I thought, ‘Oh my God. This is gonna work! This is gonna work!’” Hastings founded Netflix in 1997.

5. A Bug Collection Prompted Pinterest

As a child growing up in Iowa, Pinterest founder Ben Silbermann was obsessed with collections. A particular fascination was with bugs, and Silbermann would dry, label, and pin them inside shadow boxes: “I really liked insects,” he told Fast Company in 2012. “All kinds: flies, grasshoppers, weevils.”

When it came time for college, he entered as a premed major—both his parents are doctors—but during junior year, he decided to go into technology. After graduating from Yale, Silbermann worked for Google but left to follow his dream of designing apps.

“I had a bunch of ideas when I first left,” he told Chris Dixon at SXSW in 2012. Eventually he returned to his obsession with collections: “These things you collect say so much about who you are and what you’re interested in,” he said. “There was nothing on the web that made it easy to show off those kinds of collections.” Working with partner and college friend Paul Sciarra, he launched Pinterest in 2009.

6. Being Low On Rent Money Started Airbnb

In 2007, San Francisco entrepreneurs Joe Gebbia and Brian Chesky were looking to launch the next big thing. The pair who met while students at Rhode Island School of Design had lots of ideas but not enough rent money. After learning that a local design conference had the city’s hotels fully booked, they decided to rent three airbeds on their living-room floor to help them make the rent payment. Six days later three guests each paid $80 each a night. That’s when they realized they were onto something.

“People have said it’s the worst idea that ever worked,” Chesky said at Vanity Fair’s New Establishment Summit in October 2014. “It wasn’t supposed to be the big idea; it was supposed to be a way to pay the rent while we thought up the big idea.” Airbnb was launched in 2008.

7. Having The Flu Helped Craft Yelp

Former PayPal employees Jeremy Stoppelman and Russel Simmons joined the business incubator MRL Ventures in 2004 to launch their idea for an email-based referral service they called Yelp. Initially the concept failed to attract investors, then Stoppelman caught the flu and had a difficult time finding a recommendation for a local doctor.

“Back then there was very little information on the Internet; it was frustrating,” Stoppelman told the Wall Street Journal in 2012. “We realized the best way to find a doctor, or other services, was by word of mouth.”

Changing the model from email- to Internet-based, the pair relaunched the concept in 2005. By 2007, the site had 6 million monthly visitors.

8. An Unbalanced Checkbook Produced Mint.com

Aaron Patzer, founder of Mint.com, had always been a stickler for a balanced checkbook. Since he was a teenager, he used Quicken and Microsoft Money to manage his personal and business finances and would diligently review his accounts every Sunday night. After college, he got busy working at a Silicon Valley startup and weeks passed before he could check his balances.

“Finally, I got back into it and Microsoft Money had downloaded about 500 transactions, but only 20% had been categorized,” he told Mashable in 2011. Frustrated, he says he felt like he could categorize these transactions much more accurately by using the Yellow Pages. So he got a database of 20 million merchants, built an algorithm, and got the categorization accuracy to about 90%.

Deciding that others would benefit from his idea, Patzer launched a public beta of Mint.com in 2007. By the end of its first year, the company had 500,000 users.