Unlike modern slot machines though, the crude barrel-slots made by Sittman and Pitt have no method of direct payout -- the bar would be the one to give out prizes to winners. A straight, for example, would probably give a player free drinks.

The simplicity and ingenuity of Sittman and Pitt's barrel-device soon caught the attention of entrepreneurs and inventors. In 1887, Charles Fey simplified this barrel-machine and turned into what we could recognize as the predecessor of modern slots. Fey reduced the number of barrels from five to three. And instead of card faces, images of fruits were used. A player only needs to line up three similar images in order to win.

The use of images instead of cards made the game accessible to all kinds of people, even those who do not know poker hands. It is this simplicity and accessibility that made slot machines a hit in casinos.

Today, slot machines have evolved from using simple roller mechanisms to using random number generators (RNGs). RNGs use a complicated mathematical algorithm to fish out true random numbers. Most casinos, even land-based ones, use RNGs to ensure fair play.

And since the advent of the World Wide Web, online versions of the slot machine have become popular. Imagine playing slots at your own home, whenever you want. The very accessible game has gained more mileage because of the Internet. And now, online slot games are a hit when it comes to Internet gambling.