A lever is a board or bar that rests on a turning point. Think of a lever as a stick that is free to pivot or move back and forth at a certain point. Levers are probably the most common simple machines because just about anything that has a handle on it has a lever attached. The point on which the lever moves is called the fulcrum. By changing the position of the fulcrum, you can gain extra power with less effort. A good example of a lever is a see-saw.

Levers are divided into three different classes. They are first-class lever, second-class lever, and third-class lever.

1. A first-class lever is a stick where the fulcrum is between the weight and the energy moving the weight (for example, your hands).

Some examples of first-class levers are see-saws, crowbars, pliers, and scissors.

2. A second-class lever is a stick where the fulcrum is at one end of the stick, and you push on the other end, and the weight is in the middle of the stick.

Some examples of second-class levers are doors, staplers, wheelbarrows, and can openers.

3. A third-class lever is a stick where the fulcrum is at one end of the stick, you push on the middle, and the weight is at the other end of the stick. With a third-class lever, you have to put in more energy than you would just lifting the weight, but you get the weight to move a longer distance in return.

Some examples of third-class levers are brooms, hoes, fishing rods, and baseball bats.

A pulley is a wheel and axle with a rope or chain attached. A pulley makes work seem easier because it changes the direction of motion to work with gravity. Perhaps you have to lift a heavy load, like a bale of hay, up to the second floor of a barn. You could tie a rope to the bale of hay, stand on the second floor, and pull it straight up. Or you could put a pulley at the second floor, stand at the first floor, and lift the bale of hay by pulling straight down. It would be the same exact amount of work in either case, but the action of pulling down feels easier because you're working with the force of gravity.

A wheel and axle is really two machines in one because you can use each part in different ways. The first way is to roll something along. Wheels help you move an object across the ground because they cut down on the amount of friction between what you're trying to move and the surface you're pulling it against. (The axle is the object that attaches the wheel to the object it's moving.)

The second way of using a wheel is like a lever in the round. A door knob or a faucet on a sink are really round levers, and the "fulcrum" is in the middle where the axle turns. Imagine if a door knob was replaced with a little rod. It would be much harder to open the door!

The inclined plane is the simplest of simple machines because to make it work, nothing moves. You do the moving! Another name for an inclined plane is a ramp. A ramp works by helping you lift things more easily up to a higher level. It can be really difficult to carry a box of equipment up a ladder. But carrying that same box up a staircase is an easier job, and carrying it up a smooth ramp is even easier.

A wedge is a simple machine that cuts or pierces other objects. A wedge is really an inclined plane turned on its side. But instead of helping you move things to a higher level, a wedge helps you push things apart. The blades of a knife or a shovel are both wedges.

A wedge can also be round, like the tip of a nail or the tines on a fork. A wedge works just like a ramp: The narrower the wedge (or the sharper the point of a wedge), the easier it is drive it in and push things apart.

A screw is really an inclined plane in the round with a wedge at the tip. Think of a typical screw. The wedge is the pointed end. The inclined plane is the thread that wraps around the screw. Screws are used in many different places to hold things together.

Basically, a screw is like the ramp — and the width of the thread is like the angle of an inclined plane. The wider the thread of a screw, the harder it is to turn it.