There are only two kinds of "stupid" ideas in business: The kind that aren't pursued and the kind that don't make money. Does it really matter what anyone thinks of the pet rock when the "inventor" is laughing his way to the bank? The reality is that you don't have to create a way to change the world to get rich; you just have to find a demographic and market your product correctly.

There are a lot of would-be inventors in the world who never buckle down and make it happen. There are also a lot of business niches that you wouldn't imagine had a big enough following to work--but they do. Here are some of the allegedly "stupidest" ideas in history that made the creator or founders millionaires and billionaires. Maybe next time you'll think twice before brushing off an idea as ludicrous.

1. The Snuggie

The poor Snuggie is still made fun of, even when it now comes in leopard print and a variety of personalized options. Who wouldn't want a blanket with sleeves? There's also the knockoff Slanket, which has also made the founders filthy rich. It's an embarrassing product to admit you have, but over 20 million people in the U.S. alone bought it during its first year--and at up to $20 per product, that's no small sum. Today, they're available for pets and kids too, and are a go-to holiday gift.

2. Koosh balls

Remember these? They're a ball (kind of). Inventor Scott Stillinger wanted to create a ball that his kids could catch more easily and did so by tying some rubber bands together around a rubber core. The Koosh was born and sold millions. Stillinger sold his company for an undisclosed amount, and Koosh was then sold to Hasbro for over $100 million in 1997.

3. Singing fish

Who would have thought a plastic fish that sings would be worth so much? There was a time when almost everyone had a Big Mouth Bill Bass, Rocky Rainbow Trout, or a similar gadget. It was a prime gag gift and stores report selling hundreds per hour during the peak singing-fish era of the early aughts. There were over one million sold in 2000, making a tiny Texas novelty toy company that created it rich.

4. Shake Weight

Can you really shake your way to weight loss and a more toned body? Most trainers say no, and considering how inappropriately suggestive this device is, it's surprising that anyone under 18 is allowed to buy one. It is a popular gag gift, though, and once the YouTube ad went viral with over four million views, it ended up making the creators around $40 million.

5. Chia Pet

Everyone had a Chia Pet in the '80s and '90s and the original ram is still available today. Surprisingly, it's the gag gift that will never die and continues to rake in millions for the company. It's ridiculous, easy, and very cost effective to manufacture. In other words, it's the perfect invention to keep on giving.

As long as there's a market for something and you sell it right, there's no such thing as a stupid idea. However, this doesn't just apply to tangible items and products you might find in in-flight magazines. It's also applicable to websites. Thinking back, who would have thought a digital university yearbook would make a college kid one of the richest people in the world? Who could have imagined a blog could make people rich (hello, HuffPost). Thinking outside the box and knowing what people want is key.