When you think of a machine, you probably think of something big like a washing machine or car or bulldozer. Those big machines are made up of smaller parts called simple machines. Simple machines are made of one or two parts, and they help us do work like making things move. There are several different kinds of simple machines:
Gears are also important simple machines. A gear is just a wheel with teeth, sometimes called a cog. To do any work with a gear, you need to have at least two cogs with their teeth fitting into each other. Because the teeth fit together, when you turn one gear, the other one turns too! Gears come in many different sizes, which help them do work. If you connect one big gear with a small one, you can turn the big gear slowly and it will make the little gear turn quickly. Since it takes less energy to turn the big gear slowly than it would to turn the little one quickly, you are saving energy and making work easier by using gears.
One gear can make another one turn faster, but it can also make it turn in a different direction. When you turn a big gear to the right, the little one will move to the left. This is good for many machines where the direction needs to change in order for the machine to work.
When two or more gears are connected, it's called a 'gear train.' The gear that you turn is called the 'driver' and the last gear that you are trying to move is called the 'driven' gear. Sometimes there will be several gears in between the driver and the driven—those are called 'idlers.'
Gears are everywhere! Some rides at an amusement park or fair use large gears which make them spin in circles (like the spinning teacup ride at Disneyland). Huge wind turbines use gears to convert the slow motion of the blades into much faster motion to make electricity.
Gears are in small things, too, like tiny music boxes or remote controlled cars. They are used in clocks to make the minute and the hour hand go around. They are used in motors: big car motors, and little motors that make your toys move. Most of the time they are hidden away inside so you can't see them, but if you look at a can opener, you will be able to see two gears. Turning the handle will make one of the gears move, and the other one will move in the opposite direction. You can also see gears on a bicycle. With some bikes, the wheels will move at different speeds when you pedal because of gears connected by a chain.